Refraction of Light MCQs Quiz | Class 10

Understanding Refraction of Light

Refraction of light is a fundamental phenomenon in optics that describes the bending of light as it passes from one transparent medium to another. This change in direction occurs because light travels at different speeds in different media.

Meaning and Basic Concepts

At its core, refraction is the change in the direction of a light ray as it obliquely crosses the boundary separating two different optical media. For example, when light travels from air into water, or from water into glass, its path bends.

• Optical Medium: A material through which light can pass. Examples include air, water, glass, and diamond.
• Normal: An imaginary line drawn perpendicular to the surface at the point where the incident light ray strikes it. All angles (incidence, refraction) are measured with respect to the normal.
• Angle of Incidence (i): The angle between the incident ray and the normal.
• Angle of Refraction (r): The angle between the refracted ray and the normal.

Bending of Light

The extent to which light bends depends on the refractive indices of the two media and the angle of incidence. The key principle here is that light changes its speed. When light enters an optically denser medium (like from air to water), its speed decreases, and it bends towards the normal. Conversely, when it enters an optically rarer medium (like from water to air), its speed increases, and it bends away from the normal.

• Optically Denser Medium: A medium in which the speed of light is less.
• Optically Rarer Medium: A medium in which the speed of light is greater.
• No Bending: If the light ray strikes the surface normally (perpendicularly), it passes undeviated, even though its speed changes.

Laws of Refraction (Snell’s Law)

Refraction is governed by two main laws:

1. The incident ray, the refracted ray, and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane.
2. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant, for the light of a given color and for the given pair of media. This law is also known as Snell’s Law. Mathematically, it is expressed as:
   sin(i) / sin(r) = constant = n21
   where n21 is the refractive index of the second medium with respect to the first.

Refractive Index

The refractive index (n) of a medium is a measure of how much the speed of light is reduced when passing through that medium. It is defined as the ratio of the speed of light in vacuum (c) to the speed of light in the medium (v):

n = c / v

A higher refractive index means light travels slower in that medium and bends more when entering it from a vacuum or air.

Refractive Indices of Common Materials

Everyday Examples of Refraction

• Apparent Depth: A swimming pool appears shallower than it actually is due to the refraction of light coming from the bottom of the pool.
• Bending of a Spoon: A spoon or pencil partially immersed in water appears bent at the water surface.
• Twinkling of Stars: The light from stars undergoes multiple refractions due to varying refractive indices of Earth’s atmosphere.
• Mirages: Occur due to total internal reflection and refraction caused by temperature gradients in the air.

Quick Revision Summary

• Refraction is the bending of light as it passes from one medium to another.
• It is caused by the change in the speed of light.
• Light bends towards the normal when going from rarer to denser medium, and away from the normal when going from denser to rarer medium.
• Snell’s Law: sin(i) / sin(r) = constant (refractive index).
• Refractive index (n) = Speed of light in vacuum (c) / Speed of light in medium (v).
• The optical density of a medium is directly related to its refractive index.

Practice Questions

1. A ray of light traveling in air enters obliquely into water. Which of the following statements is true?
   1. The light ray bends towards the normal.
   2. The light ray bends away from the normal.
   3. The light ray does not bend.
   4. The light ray reflects back into the air.

   Answer: a) The light ray bends towards the normal.

2. Which phenomenon is responsible for the twinkling of stars?
   1. Reflection of light
   2. Dispersion of light
   3. Refraction of light
   4. Scattering of light

   Answer: c) Refraction of light.

3. If the angle of incidence is 0 degrees, what will be the angle of refraction?
   1. 0 degrees
   2. 90 degrees
   3. 45 degrees
   4. Depends on the medium

   Answer: a) 0 degrees.

4. The refractive index of glass is 1.5. If the speed of light in vacuum is 3 x 10^8 m/s, what is the speed of light in glass?
   1. 1.5 x 10^8 m/s
   2. 2.0 x 10^8 m/s
   3. 3.0 x 10^8 m/s
   4. 4.5 x 10^8 m/s

   Answer: b) 2.0 x 10^8 m/s (v = c/n = 3 x 10^8 / 1.5 = 2 x 10^8 m/s).

5. Which of the following has the highest optical density?
   1. Air
   2. Water
   3. Glass
   4. Diamond

   Answer: d) Diamond.