CG Board Class 8 Science Chapter 9 Solutions: Refraction of Light (प्रकाश का अपवर्तन) – 2026-27

When light travels from an optically rarer medium (air) to an optically denser medium (glass), its speed decreases. Consequently, the refracted ray bends towards the perpendicular normal (i > r).

When light travels from an optically denser medium (glass) to an optically rarer medium (air), its speed increases. Consequently, the refracted ray bends away from the perpendicular normal (r > i).

When a ray of light moves from a denser medium to a rarer medium, it speeds up and deviates away from its original path by bending away from the normal. The angle of refraction (r) becomes greater than the angle of incidence (i).

If the light ray deviates towards the normal upon entering medium 2 from medium 1, it indicates that

1. Medium 1 is an optically rarer medium.
2. Medium 2 is an optically denser medium.

A light ray falling perpendicularly (at an angle of incidence i = 0°) on the surface of an optical medium passes straight through without any deviation or bending, continuing along the normal in the same straight direction.

Since glass is optically denser than water

1. The refractive index of air to glass (μ_g ≈ 1.5) is more.
2. The refractive index of air to water (μ_w ≈ 1.33) is less.

(Reason: Optical density is directly proportional to refractive index; higher optical density results in a higher refractive index).

The minimum distance at which an object can be clearly viewed by a normal human eye without strain is 25 cm.

In a human eye, the real and inverted image of an object is formed on the light-sensitive innermost membrane called the retina. (The image formed specifically at the yellow spot on the retina is the clearest).

Shortsightedness (Myopia):** It is an eye defect in which a person can see nearby objects clearly but cannot see distant objects distinctly. In this defect, the image of a distant object is formed in front of the retina rather than on it.

• Correction: It is corrected by using spectacles fitted with a suitable double concave lens (diverging lens). The concave lens diverges incoming parallel light rays so that they focus precisely on the retina.

Longsightedness (Hypermetropia):** It is an eye defect in which a person can see distant objects clearly but cannot see nearby objects distinctly. Due to increased focal length or eye strain, the image of a nearby object is formed beyond (behind) the retina.

• Correction: It is corrected by using spectacles fitted with a suitable convex lens (converging lens). The convex lens converges the light rays so that they focus properly on the retina.

(a) 1.5**

(b) Optical centre**

(c) Refraction of light**

(a) At 2F**

When a ray of light passes obliquely from one transparent optical medium to another, it undergoes a change in its direction and speed at the boundary separating the two media. This phenomenon of bending of light rays due to a change in the medium is called refraction of light.

Glass is an optically denser medium compared to water, which is an optically rarer medium (μ_glass ≈ 1.5 > μ_water ≈ 1.33). When a light ray enters water from glass (denser to rarer medium), it speeds up and deflects away from the normal.

Definition:** Refractive index (μ) is a measure of the optical density of a medium that indicates how much a ray of light bends or changes its speed when passing from one medium to another.

• Formula: It is defined as the ratio of the velocity of light in vacuum (or air) to the velocity of light in that specific medium.

Refractive Index (μ) = (Velocity of light in vacuum / air (c))/(Velocity of light in the particular medium (v))

Ray Diagram Description & Features:**

When an object (AB) is placed between the optical centre (O) and principal focus (F) of a convex lens

1. Incident Rays: One ray parallel to the principal axis refracts through focus F. A second ray passing through the optical centre O goes straight without deviation.
2. Diverging Refracted Rays: On the other side, these rays diverge. When extended backwards (dotted lines), they meet at a point behind the object.
3. Image Characteristics (A’B’):

• Position: On the same side of the lens as the object.
• Nature: Virtual and erect.
• Size: Magnified (larger than the object).
• Diagram Structure Representation:

`text

A’ |

| A

———-|——|——-F——-O——-F——-2F—— (Principal Axis)

| B |

B’ | | (Convex Lens)

|<--- Virtual Image --->|

Longsightedness (Hypermetropia):** It is a defect of vision in which a person can see distant objects clearly but cannot see nearby objects distinctly.

• Cause: It occurs due to increase in focal length of the eye lens or shortening of the eyeball, causing the image of nearby objects to form behind the retina.
• Correction: It is rectified using spectacles fitted with a convex lens of appropriate focal length. The convex lens converges incoming rays before they enter the eye, shifting the focus forward onto the retina.
• Diagram Description:

1. Defective Eye: Parallel/diverging rays from a nearby object focus beyond the retina.
2. Corrected Eye: A convex lens placed in front of the eye bends the light rays inward so that after passing through the eye lens, they focus exactly on the retina.

`text

(a) Hypermetropic Eye (Defect)

Light Rays —> [ Eye Lens ] —> Image formed BEHIND Retina

(b) Rectification using Convex Lens

Light Rays —> ( Convex Lens ) —> [ Eye Lens ] —> Image formed ON Retina

Activity (Refraction through a Glass Slab):**

1. Setup: Fix a white sheet of paper flat on a wooden drawing board using pins. Place a rectangular glass slab on the paper and trace its border outline (PQRS).
2. Incident Ray & Normal: Draw an inclined line AB meeting surface PQ at point O (point of incidence). At point O, draw a perpendicular line M_1M₂ representing the normal to the surface.
3. Refraction & Outgoing Ray: Place the glass slab back. Direct light rays along AB using pins or a light streak. Mark the points C and D where the ray emerges out of surface RS at point P.
4. Joining the Rays: Remove the glass slab and join point O to point P to obtain the refracted ray OP inside the glass slab.
5. Observation & Conclusion:

All the drawn lines: the incident ray (AB), the normal (M_1M₂) at the point of incidence O, and the refracted ray (OP) lie completely flat on the same two-dimensional plane of the paper sheet. This verifies the first law of refraction.