Class 10 Science Chapter 10 Activities | The Human Eye and the Colourful World
Activity 10.1: Refraction of Light Through a Triangular Glass Prism
Tracing Incident, Refracted, and Emergent Rays Across a Glass Prism
Experimental Procedure
- Fix a sheet of white paper on a drawing board using drawing pins and place a glass prism on it resting on its triangular base.
- Trace the outline of the prism using a pencil and draw a straight line \(PE\) inclined to one of the refracting surfaces (\(AB\)).
- Fix two pins at points \(P\) and \(Q\) on line \(PE\), look through the other face \(AC\) for their images, and fix two more pins at \(R\) and \(S\) in a straight line with those images.
- Remove the prism, join and produce lines to meet boundaries at \(E\) and \(F\), draw normals, and mark angles \(i\), \(r\), and \(e\).
Observations & Optical Mechanism
- First Interface (\(AB\)): Light enters from air to glass and bends towards the normal.
- Second Interface (\(AC\)): Light enters from glass to air and bends away from the normal.
- Angle of Deviation: The peculiar shape of the prism makes the emergent ray bend at an angle (\(\angle D\)) to the direction of the incident ray.
Q1: What is the angle between the two lateral faces of a glass prism called?
Answer: The angle of the prism.
Q2: How does a ray of light bend when entering from air to glass at the first surface of a prism?
Answer: It bends towards the normal.
Q3 (Application Based): What defines the angle of deviation (\(\angle D\)) in a prism experiment?
Answer: It is the angle at which the emergent ray bends relative to the direction of the incident ray due to the prism’s shape.
Q4 (Additional Practice): How does the bending of light at the prism surfaces compare to that in a rectangular glass slab?
Answer: Unlike a glass slab where parallel surfaces cause the emergent ray to be parallel to the incident ray, a prism’s inclined surfaces cause a distinct net angle of deviation.
Activity 10.2: Dispersion of White Light by a Glass Prism
Splitting Sunlight into a Spectrum of Colours
Experimental Procedure
- Take a thick sheet of cardboard and make a small hole or narrow slit in its middle to allow a narrow beam of sunlight.
- Take a glass prism and allow the light from the slit to fall on one of its faces.
- Turn the prism slowly until the emerging light appears on a nearby screen.
- Observe the band of distinct colours formed on the screen.
Observations & Optical Mechanism
- Spectrum Formation: The prism splits the incident white light into a band of seven colours: Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR).
- Differential Bending: Different colours bend through different angles; red light bends the least while violet bends the most.
- Dispersion: The splitting of light into its component colours is called dispersion.
Q1: What is the sequence of colours seen on the screen when white light passes through a prism?
Answer: Violet, Indigo, Blue, Green, Yellow, Orange, and Red (remembered via the acronym VIBGYOR).
Q2: Which colour of light bends the most and which bends the least during dispersion through a prism?
Answer: Violet light bends the most, while red light bends the least.
Q3 (Application Based): How did Isaac Newton prove that sunlight is composed of seven colours?
Answer: He placed a second identical prism in an inverted position relative to the first prism, allowing all spectrum colours to pass through, which recombined them into a beam of white light.
Q4 (Additional Practice): What is a natural spectrum appearing in the sky after a rain shower called?
Answer: A rainbow.
Q5 (Extra Concept): What role do tiny water droplets play in the formation of a natural rainbow?
Answer: They act like small prisms that refract, disperse, internally reflect, and refract sunlight again.
Core Concepts: The Human Eye and Vision Defects
Anatomy, Accommodation, and Corrective Lenses
Structural & Functional Overview
- Cornea & Iris: Light enters through the transparent cornea; the iris regulates the size of the pupil, controlling the amount of light entering the eye.
- Retina & Accommodation: The eye lens forms an inverted real image on the light-sensitive retina. Ciliary muscles adjust the curvature and focal length of the eye lens to focus objects at varying distances.
- Near and Far Points: The near point (least distance of distinct vision) for a normal young adult is about \(25\text{ cm}\), and the far point is infinity.
Refractive Defects and Corrections
- Myopia (Near-sightedness): Distant objects appear blurred because images form in front of the retina due to excessive curvature or elongated eyeball; corrected using a concave lens.
- Hypermetropia (Far-sightedness): Nearby objects appear blurred because images form behind the retina due to long focal length or small eyeball; corrected using a convex lens.
- Presbyopia: Loss of accommodation due to aging and weakened ciliary muscles; often corrected with bi-focal lenses.
Q1: What is meant by the power of accommodation of the eye?
Answer: The ability of the eye lens to adjust its focal length to focus on both near and distant objects clearly.
Q2: Why do stars twinkle while planets do not?
Answer: Stars are distant point-sized light sources whose light fluctuates due to atmospheric refraction, causing a twinkling effect. Planets are closer, extended sources whose individual point variations average out to zero.
Q3 (Application Based): Why is the Sun visible about 2 minutes before actual sunrise and 2 minutes after actual sunset?
Answer: Due to atmospheric refraction of sunlight through layers of gradually changing refractive index.
Q4 (Additional Practice): Explain why the clear sky appears blue during the day.
Answer: Fine air molecules scatter light of shorter wavelengths (blue end) much more strongly than longer wavelengths (red end) via the Tyndall effect.
Q5 (Extra Concept): Why are ‘danger’ signal lights colored red?
Answer: Because red light has the longest wavelength and is least scattered by fog or smoke, allowing it to be seen clearly over long distances.
