Class 10 Science Chapter 9 Light – Reflection and Refraction Activities

Activity 9.1: Observing Images with a Shining Spoon

Comparing Image Characteristics on Concave and Convex Curved Surfaces

Experimental Procedure

• Take a large shining spoon and try to view your face in its inner curved surface.
• Observe whether the image is smaller or larger, and move the spoon slowly away from your face to note changes.
• Reverse the spoon to use its outer bulging surface and repeat the observations.
• Compare the distinct optical characteristics of the images formed by both curved surfaces.

Observations & Optical Mechanism

• Inner Surface (Concave): Acts as a concave mirror, forming an inverted real image when far, and an erect, magnified virtual image when close.
• Outer Surface (Convex): Acts as a convex mirror, consistently forming a diminished, erect virtual image regardless of distance.
• Inference: Curved mirror geometry dictates whether light rays converge or diverge, altering image size, orientation, and nature.

Q1: How do the images formed by the inner and outer surfaces of a shining spoon differ?

Answer: The inner (concave) surface can produce magnified, inverted, or erect virtual images depending on distance. The outer (convex) surface always produces a diminished, erect virtual image.

Q2: What is a spherical mirror? Distinguish between a concave mirror and a convex mirror.

Answer: A spherical mirror is a mirror whose reflecting surface forms part of a sphere. A concave mirror has its reflecting surface curved inwards, whereas a convex mirror has its reflecting surface curved outwards.

Q3 (Application Based): Why are convex mirrors preferred as rear-view mirrors in automobiles?

Answer: Convex mirrors always provide an erect, diminished image and have a much wider field of view because they curve outwards, allowing drivers to view a vast traffic area behind them.

Q4 (Application Based): State the relation between the radius of curvature (R) and focal length (f) of a spherical mirror.

Answer: The radius of curvature is equal to twice the focal length:

\(R = 2f\)

Q5 (Additional Practice): What happens to the size of the image formed by a concave mirror when you move your face very close to its inner surface?

Answer: The image transitions into an enlarged, erect virtual image that appears magnified behind the mirror surface.

Q6 (Additional Practice): Can a convex mirror form a real image under any specific experimental condition with real objects?

Answer: No. A convex mirror diverges parallel or incident rays, meaning real rays never actually intersect in front of it; hence, it can only form virtual images for real objects.

Q7 (Extra Concept): Are the laws of reflection applicable to spherical surfaces like a shining spoon?

Answer: Yes, the laws of reflection are applicable to all types of reflecting surfaces, including spherical surfaces.

Q8 (Advanced Mastery): According to New Cartesian Sign Convention, what is the sign of the focal length of a concave mirror versus a convex mirror?

Answer: The focal length of a concave mirror is negative, while the focal length of a convex mirror is positive.

Activity 9.2: Determining Focal Length Using Sunlight

Focusing Solar Radiation to Ignite Paper and Estimate Focal Length

Experimental Procedure

• Hold a concave mirror and direct its reflecting surface toward the distant Sun.
• Hold a paper sheet in front of the mirror and adjust its distance back and forth until a sharp, bright spot of light is obtained.
• Hold the mirror and paper steady in this position for a few minutes and observe.

Observations & Optical Mechanism

• Observation: The paper begins to smoke and eventually catches fire.
• Optical Mechanism: Sunlight consists of parallel rays that converge at the principal focus after reflection from a concave mirror, generating intense thermal concentration.
• Focal Length Estimation: The measured distance between the mirror pole and the sharp bright spot gives the approximate focal length (f) of the concave mirror.

Q1: Why does the paper sheet catch fire when placed at the focus of a concave mirror facing the Sun?

Answer: The concave mirror converges parallel sun rays onto a sharp focal point, concentrating solar thermal energy intensely enough to ignite the paper.

Q2: Define the principal focus of a concave mirror.

Answer: The principal focus of a concave mirror is a point on its principal axis where rays parallel to the principal axis meet or intersect after reflection from the mirror.

Q3 (Application Based): If the radius of curvature of a spherical mirror is 20 cm, what is its focal length?

Answer: Using the relation

\(f = \frac{R}{2}\)

, the focal length is \(20\text{ cm} / 2 = 10\text{ cm}\).

Q4 (Additional Practice): Why would a convex mirror fail to ignite paper when pointed toward the Sun in the same manner?

Answer: A convex mirror diverges parallel rays instead of focusing them, preventing any concentration of thermal energy.

Q5 (Additional Practice): What term is given to the diameter of the circular outline of a spherical mirror’s reflecting surface?

Answer: The aperture of the spherical mirror.

Q6 (Extra Concept): What safety precaution is explicitly mentioned before looking into a mirror reflecting sunlight in Activity 9.2?

Answer: Do not look at the Sun directly or even into a mirror reflecting sunlight because it may damage your eyes.

Q7 (Advanced Mastery): How does the intensity of the focused bright spot change if a mirror with a larger aperture is used?

Answer: A larger aperture collects more light rays, increasing thermal concentration at the focus and causing ignition to happen faster.

Activity 9.3: Image Formation by a Concave Mirror

Mapping Object Positions and Screen Image Characteristics

Experimental Procedure

• Set up a concave mirror on a stand and mark reference lines corresponding to pole P, focus F, and centre C on a table.
• Place a burning candle beyond C and move a paper screen to capture a sharp image.
• Record nature, position, and relative size of the image.
• Repeat for object positions at C, between C and F, at F, and between P and F.

Observations & Optical Mechanism

• Real Images: Formed on the screen when the object is placed at or beyond F (inverted and real).
• Virtual Image: When placed between P and F, no screen image forms; an enlarged, erect virtual image is seen inside the mirror.
• Inference: Image characteristics vary systematically according to object distance relative to F and C.

Q1: Where should an object be placed in front of a concave mirror to obtain a real image of the exact same size as the object?

Answer: At the centre of curvature (C).

Q2: State the mirror formula relating object distance (u), image distance (v), and focal length (f).

Answer:

\(\frac{1}{v} + \frac{1}{u} = \frac{1}{f}\)

Q3 (Application Based): Write the expression for magnification (m) produced by a spherical mirror in terms of object and image heights or distances.

Answer:

\(m = \frac{h’}{h} = -\frac{v}{u}\)

Q4 (Additional Practice): What does a negative sign in magnification indicate about the image?

Answer: It indicates that the image is real and inverted.

Q5 (Additional Practice): If an object is placed at infinity in front of a concave mirror, where is the image formed?

Answer: At the principal focus (F), highly diminished and point-sized.

Q6 (Extra Concept): What happens when the object is placed at the focus (F) of a concave mirror?

Answer: The image is formed at infinity and would not be captured on a practical screen.

Q7 (Advanced Mastery): Why can virtual images formed by concave mirrors not be captured on a screen?

Answer: Because virtual images are formed where reflected light rays only appear to diverge from, meaning no actual light energy strikes the screen location.

Activity 9.4: Drawing Ray Diagrams for Concave Mirrors

Verifying Image Positions for All Object Locations

Experimental Procedure

• Draw neat ray diagrams for each position of the object corresponding to Table 9.1 (At infinity, Beyond C, At C, Between C and F, At F, Between P and F).
• Choose any two standard incident rays (parallel to principal axis, passing through F or C, or obliquely to P) to locate the image point.
• Compare the graphical results with experimental screen observations.

Observations & Optical Mechanism

• Ray Intersection: The intersection point of reflected rays determines the precise location, size, and nature of the image.
• Consistency: Graphical ray tracing matches the experimental screen findings across all object positions.

Q1: What happens to the position of the image formed by a concave mirror as the object is moved from infinity towards the pole?

Answer: The image moves progressively away from the focus towards infinity as the object approaches the mirror.

Q2 (Application Based): Where is the image formed if an object is placed at the principal focus (F) of a concave mirror?

Answer: At infinity, forming a highly magnified real image.

Q3 (Additional Practice): What happens to a ray passing through the centre of curvature (C) of a concave mirror after reflection?

Answer: It is reflected back along the same path because it strikes the surface normally 90°.

Q4 (Extra Concept): How do a ray parallel to the principal axis and a ray passing through the principal focus behave after reflection in a concave mirror?

Answer: A ray parallel to the principal axis passes through the principal focus after reflection, whereas a ray passing through the focus emerges parallel to the principal axis.

Q5 (Advanced Mastery): Why are any two rays sufficient to locate an image in a ray diagram?

Answer: Because all infinite light rays emanating from a single object point converge at the exact same unique intersection point after reflection.

Activity 9.5: Image Formation by a Convex Mirror

Observing Pencil Images in a Convex Mirror

Experimental Procedure

• Hold a convex mirror in one hand and an upright pencil in the other.
• Observe the image of the pencil inside the convex mirror and check whether it is erect or inverted, diminished or enlarged.
• Slowly move the pencil away from the mirror and observe how the image size changes.

Observations & Optical Mechanism

• Image Nature: The image is always virtual, erect, and diminished regardless of the pencil’s distance.
• Size Variation: As the pencil is moved further away, the image becomes progressively smaller and moves closer to the focus behind the mirror.

Q1: What is the nature of the image formed by a convex mirror for any real object position?

Answer: Always virtual, erect, and diminished.

Q2 (Application Based): Why are convex mirrors used as security mirrors in department stores?

Answer: Because they provide an erect image and a very wide field of view.

Q3 (Additional Practice): Where is the principal focus located in a convex mirror?

Answer: Behind the mirror (virtual focus).

Q4 (Extra Concept): State whether the image moves closer to or farther away from the focus as an object is moved away from a convex mirror.

Answer: The image moves closer to the focus behind the mirror as the object is moved away.

Q5 (Advanced Mastery): What is the sign of magnification (m) for a convex mirror, and what does it signify?

Answer: The magnification is always positive, signifying that the image formed is virtual and erect.

Activity 9.6: Full-Length Images in Mirrors

Comparing Field of View and Full-Length Reflections of Distant Objects

Experimental Procedure

• Observe a distant tall object, such as a tree or building, in a plane mirror of varying sizes.
• Repeat the observation using a concave mirror and then a convex mirror.
• Determine which mirror successfully displays a full-length image of the tall object.

Observations & Optical Mechanism

• Plane Mirror: Can only show a full image if the mirror size is at least half the height of the object and placed at a suitable distance.
• Convex Mirror: Easily displays a full-length, erect, diminished image of a massive tall building due to its outward curvature and wide field of view.

Q1: Which type of spherical mirror is capable of forming a full-length diminished image of a tall distant building?

Answer: A convex mirror.

Q2 (Application Based): Where is a famous giant convex mirror installed in India to view the full Taj Mahal?

Answer: Fitted in a wall of the Agra Fort facing the Taj Mahal.

Q3 (Additional Practice): What two key features make convex mirrors ideal as vehicle side-view mirrors?

Answer: They always produce erect images and provide a much wider field of view than plane mirrors.

Q4 (Extra Concept): To view the full image of the Taj Mahal distinctly from the Agra Fort wall, where should an observer stand?

Answer: Suitably at the terrace adjoining the wall.

Q5 (Advanced Mastery): Why would a plane mirror fail to show the full image of a tall building when viewed from a short distance?

Answer: Because a plane mirror produces an image of the same size as the object, requiring mirror dimensions proportional to the entire building height.

Activity 9.7: Picking up a Coin in Water

Attempting to Grab a Submerged Coin

Experimental Procedure

• Place a coin at the bottom of a bucket filled with water.
• Position your eye to the side above water and try to pick up the coin in one go.
• Note whether you succeeded on your first attempt.

Observations & Optical Mechanism

• Observation: Most people fail to pick up the coin in one go.
• Optical Mechanism: Light rays from the coin refract at the water-air interface, creating an apparent elevated position.

Q1: Why is it difficult to pick up a coin submerged in water on the first try?

Answer: Due to refraction, the coin appears raised above its actual position, tricking hand-eye coordination.

Q2 (Application Based): Does light travel faster in water or in air?

Answer: Light travels faster in air than in water, causing the apparent depth to be shallower than the real depth.

Q3 (Additional Practice): What phenomenon causes the bottom of a water tank to appear raised?

Answer: Refraction of light.

Q4 (Extra Concept): Why did you not succeed in picking up the coin in one go during this experiment?

Answer: Because the apparent position of the coin differs from its actual physical location due to the refraction of light at the water surface.

Q5 (Advanced Mastery): How does the refractive index of water affect the apparent depth compared to real depth?

Answer: Apparent depth is equal to real depth divided by the refractive index \((\text{Apparent Depth} = \frac{\text{Real Depth}}{n}\)).

Activity 9.8: Reappearing Coin in a Bowl

Coin Visibility Upon Pouring Water into a Shallow Bowl

Experimental Procedure

• Place a coin in a shallow bowl and move away until the coin just disappears from sight.
• Have a friend pour water gently into the bowl without disturbing the coin.
• Keep looking from your fixed position and observe.

Observations & Optical Mechanism

• Observation: The coin becomes visible again when water is poured into the bowl.
• Optical Mechanism: Refraction bends light rays from the coin away from the normal as they enter the air, enabling them to reach an observer whose direct line of sight was previously blocked.

Q1: How does pouring water make a hidden coin visible again in Activity 9.8?

Answer: Refraction bends light rays from the coin upward at the water-air boundary, allowing rays to reach the observer’s eye from outside the direct straight-line obstruction.

Q2 (Application Based): What physical property of light is responsible for this phenomenon?

Answer: The change in direction (refraction) of light when traveling from an optically denser medium to a rarer medium.

Q3 (Additional Practice): Why must water be poured gently into the bowl during this activity?

Answer: To avoid disturbing the position of the coin.

Q4 (Extra Concept): From your fixed position after water is poured, does the coin appear slightly raised above its actual position?

Answer: Yes, the coin appears slightly raised above its actual position due to the refraction of light.

Q5 (Advanced Mastery): If a liquid with a higher refractive index than water is used instead, would the coin become visible from a greater distance?

Answer: Yes, a higher refractive index causes greater bending (refraction) of light, making the coin visible from an even further distance.

Activity 9.9: Observing an Ink Line Through a Glass Slab

Apparent Elevation and Bending of Lines Through Thick Glass

Experimental Procedure

• Draw a thick straight ink line on a white paper sheet placed on a table.
• Place a glass slab over the line such that its edge makes an angle with the line, and observe from the sides.
• Reposition the slab normal to the line and view from the top.

Observations & Optical Mechanism

• Observation: The portion of the line beneath the glass slab appears raised and slightly bent at the edges when viewed obliquely.
• Inference: Refraction through parallel glass faces causes apparent vertical and lateral shifts in objects viewed through dense media.

Q1: Does the ink line appear bent when viewed normally (perpendicularly) through a glass slab?

Answer: No, it does not appear bent, but the portion under the slab appears noticeably raised toward the observer.

Q2 (Application Based): Why does printing under a thick glass slab appear elevated?

Answer: Refraction bends light rays away from the normal as they exit into the air, creating a virtual image closer to the upper surface.

Q3 (Additional Practice): What happens to the appearance of the line when the glass slab is placed normal to it?

Answer: The part beneath appears raised from the top, but does not appear bent at the edges.

Q4 (Extra Concept): How does viewing the line from the top of the glass slab differ from viewing it from the sides?

Answer: From the top, the line appears raised without edge bending, whereas from the sides at an angle, it appears bent at the edges.

Q5 (Advanced Mastery): What factors govern the apparent shift of the ink line through the glass slab?

Answer: The thickness of the glass slab and its refractive index.

Activity 9.10: Refraction through a Rectangular Glass Slab

Tracing Incident, Refracted, and Emergent Rays Across Parallel Glass Faces

Experimental Procedure

• Fix a white sheet on a board, place a rectangular glass slab in the middle, and outline its boundary (ABCD).
• Fix two pins (E and F) on an inclined line directed at the upper glass face.
• View pin images through the opposite face and align two more pins (G and H) in a straight line.
• Remove slab, trace ray paths, draw normals, and measure angles of incidence, refraction, and emergence.

Observations & Optical Mechanism

• Bending at Air-Glass Interface: Light bends towards the normal as it enters the denser glass medium.
• Bending at Glass-Air Interface: Light bends away from the normal as it re-enters the rarer air medium.
• Lateral Displacement: The emergent ray is parallel to the incident ray but undergoes a slight sideways shift because opposite faces have equal and opposite bending effects.

Q1: State Snell’s law of refraction.

Answer: The ratio of SMD / sine of the angle of incidence to the sine of the angle of refraction is a constant for a given pair of media:

\(\frac{\sin i}{\sin r} = \text{constant}\)

Q2: How is the absolute refractive index \((\(n_m\))\) of a medium calculated using the speed of light?

Answer:

\(n_m = \frac{c}{v}\)

(where \(c\) is speed in air/vacuum and \(v\) is speed in medium)

Q3 (Application Based): Light enters from air into glass having a refractive index of 1.50. What is the speed of light in glass if the speed of light in vacuum is \(3 \times 10^8\text{ m s}^{-1}\)?

Answer: Using

\(v = \frac{c}{n}\)

, \(v = (3 \times 10^8\text{ m s}^{-1}) / 1.50 = 2 \times 10^8\text{ m s}^{-1}\).

Q4 (Additional Practice): Why does a light ray incident normally on a glass slab boundary pass straight without bending?

Answer: Because the angle of incidence is zero, making the angle of refraction zero according to Snell’s law.

Q5 (Extra Concept): Why is the emergent ray parallel to the incident ray in a rectangular glass slab?

Answer: Because the extent of bending of the ray of light at the opposite parallel faces (air-glass and glass-air interfaces) is equal and opposite.

Q6 (Advanced Mastery): How does lateral displacement change if the thickness of the glass slab is increased?

Answer: Lateral displacement increases as the thickness of the glass slab increases because the light travels a longer sideways path inside the denser medium.

Activity 9.11: Focusing Sunlight with a Convex Lens

Igniting Paper with Solar Radiation via a Convex Lens

Experimental Procedure

• Hold a convex lens and direct it towards the distant Sun.
• Focus the light from the Sun onto a sheet of paper to obtain a sharp bright spot.
• Hold the paper and lens steady in this position and observe.

Observations & Optical Mechanism

• Observation: The paper begins to smoke and eventually catches fire.
• Optical Mechanism: Parallel solar rays converge at the principal focus of the convex lens, producing high thermal concentration.

Q1: Why does paper burn when placed at focus of a convex lens facing the Sun?

Answer: The convex lens converges parallel sun rays onto a focal point, concentrating heat energy.

Q2 (Application Based): What does the distance between the convex lens and the bright spot on paper represent?

Answer: It gives the approximate focal length (f) of the convex lens.

Q3 (Additional Practice): What alternative name is given to a convex lens due to its action on parallel light rays?

Answer: Converging lens.

Q4 (Extra Concept): What safety warning is given when performing Activity 9.11?

Answer: Do not look at the Sun directly or through a lens while doing this activity to avoid damaging your eyes.

Q5 (Advanced Mastery): Why does a thicker convex lens have a shorter focal length than a thinner one made of the same glass?

Answer: A thicker lens has more curved surfaces, causing stronger bending (refraction) of light rays and bringing them to a focus closer to the optical centre.

Activity 9.12: Image Formation by a Convex Lens

Mapping Screen Images Across Various Object Distances from a Convex Lens

Experimental Procedure

• Set up a convex lens on a stand and mark reference lines for \(F_1\), \(2F_1\), \(F_2\), and \(2F_2\).
• Place a burning candle beyond \(2F_1\) and obtain a clear sharp image on a screen on the opposite side.
• Repeat for object positions at \(2F_1\), between \(F_1\) and \(2F_1\), at \(F_1\), and between \(F_1\) and optical centre \(O\).

Observations & Optical Mechanism

• Real vs Virtual: Real and inverted images form on the screen for objects beyond \(F_1\). A virtual erect image forms on the same side when placed within \(F_1\).

Q1: Where should an object be placed in front of a convex lens to get a real image of the same size?

Answer: At twice the focal length (\(2F_1\)).

Q2: State the lens formula relating object distance (u), image distance (v), and focal length (f).

Answer:

\(\frac{1}{v} – \frac{1}{u} = \frac{1}{f}\)

Q3 (Application Based): What type of lens is used as a magnifying glass by watchmakers?

Answer: A convex lens, when the object is placed within its focal length.

Q4 (Additional Practice): What is the SI unit of power of a lens?

Answer: Dioptre (D).

Q5 (Extra Concept): What happens to image formation when an object is placed at focus \(F_1\) of a convex lens?

Answer: The image is formed at infinity and would not be formed on a screen.

Q6 (Advanced Mastery): If a convex lens of focal length \(+10\text{ cm}\) has an object placed at \(15\text{ cm}\), calculate whether the image is real or virtual.

Answer: Since the object is placed beyond the focus (\(u = -15\text{ cm}\) where \(f = 10\text{ cm}\)), the lens forms a real and inverted image on the other side.

Activity 9.13: Image Formation by a Concave Lens

Observing Virtual and Diminished Images Through a Concave Lens

Experimental Procedure

• Take a concave lens and place it on a lens stand.
• Place a burning candle on one side of the lens and look through the other side.
• Attempt to capture the image on a screen, then observe directly through the lens.
• Move the candle away from the lens and note changes in image size.

Observations & Optical Mechanism

• Observation: No image can be caught on a screen because a concave lens produces a virtual image. When viewed directly, the image appears erect and diminished.
• Size Changes: As the candle is moved farther away, the virtual image becomes smaller and moves closer to the focus.
• Inference: A concave lens always gives a virtual, erect, and diminished image irrespective of the object position.

Q1: What is the nature of the image formed by a concave lens for any object position?

Answer: Always virtual, erect, and diminished.

Q2 (Application Based): A concave lens has a focal length of 15 cm. At what distance should an object be placed so it forms an image at 10 cm from the lens?

Answer: Using the lens formula

\(\frac{1}{v} – \frac{1}{u} = \frac{1}{f}\)

with \(v = -10\text{ cm}\) and \(f = -15\text{ cm}\), we get \(u = -30\text{ cm}\) (30 cm in front of the lens).

Q3 (Additional Practice): Why is a concave lens also known as a diverging lens?

Answer: Because it causes parallel rays of light to diverge.

Q4 (Extra Concept): What happens to the size of the image when the candle is moved away from the concave lens?

Answer: The virtual image becomes smaller and moves closer to the focus.

Q5 (Advanced Mastery): Why are concave lenses prescribed by optometrists for correcting myopia (near-sightedness)?

Answer: Concave lenses diverge incoming light rays so that they form a clear image precisely on the retina of a near-sighted eye.