What is meant by the term "refraction of light"? Explain the laws of refraction of light with the help of a diagram.

Refraction of Light

Refraction of light is the phenomenon in which a light ray bends when it passes from one transparent medium into another, changing its speed and direction. This occurs because the optical density (refractive index) of the two media differs. For example, light travels faster in air than in water or glass, causing it to bend toward or away from the normal (a line perpendicular to the boundary between the two media).

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Laws of Refraction of Light

There are two fundamental laws governing refraction:

1. First Law of Refraction

The incident ray, the refracted ray, and the normal to the boundary at the point of incidence all lie in the same plane.

2. Second Law of Refraction (Snell’s Law)

The ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction (r) is constant and equal to the relative refractive index of the two media: $$\frac{\sin i}{\sin r}=\frac{v_1}{v_2}={\mu }_{21}$$ where:

• $v_1$ = speed of light in medium 1 (e.g., air),
• $v_2$ = speed of light in medium 2 (e.g., glass),
• ${\mu }_{21}$ = refractive index of medium 2 with respect to medium 1.

Key Observations:

• If light moves from a rarer to a denser medium (e.g., air → glass), it bends toward the normal ($r<i$).
• If light moves from a denser to a rarer medium (e.g., glass → air), it bends away from the normal ($r>i$).
• The refractive index ($\mu$) of a medium is defined as: $$\mu =\frac{\text{Speed of light in vacuum}}{\text{Speed of light in the medium}}=\frac{c}{v}$$ For example, ${\mu }_{\text{glass}}\approx 1.5$ and ${\mu }_{\text{water}}\approx 1.33$.

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Practical Example

When a pencil is partially submerged in water, it appears bent at the water surface due to refraction. This happens because light rays from the pencil bend as they exit the water into the air, making the pencil seem displaced.

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Applications of Refraction

1. Lenses and Optical Instruments: Refraction is used in spectacles, microscopes, and cameras to focus light.
2. Rainbow Formation: Refraction and dispersion of sunlight in raindrops create a rainbow.
3. Optical Fibers: Light is refracted repeatedly to transmit signals over long distances with minimal loss.