Laws of Refraction MCQs Quiz | Class 10

Understanding the Laws of Refraction

Refraction is a fundamental phenomenon in optics, describing the bending of light as it passes from one transparent medium to another. This change in direction occurs due to a change in the speed of light as it travels through media of different optical densities. Understanding the laws governing refraction is crucial for comprehending how lenses work, how we see objects underwater, and various other optical instruments.

Key Concepts of Refraction

• Definition: Refraction is the change in direction of a wave passing from one medium to another or from a gradual change in the medium.
• Cause: The primary cause of refraction is the change in the speed of light as it moves from a medium of one optical density to another. If light enters a denser medium, it slows down and bends towards the normal; if it enters a rarer medium, it speeds up and bends away from the normal.
• Terms:
  • Incident Ray: The ray of light striking the boundary between two media.
  • Refracted Ray: The ray of light that has bent after entering the second medium.
  • Normal: An imaginary line perpendicular to the surface at the point of incidence.
  • 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.

The Laws of Refraction

1. First Law of Refraction

The first law of refraction states that the incident ray, the refracted ray, and the normal to the interface of the two transparent media at the point of incidence, all lie in the same plane.

This law ensures that the phenomenon of refraction is a 2D event, simplifying its analysis and prediction. It is analogous to the first law of reflection.

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

The second law of refraction, also known as Snell’s Law, provides a quantitative relationship between the angles of incidence and refraction, and the refractive indices of the two media. It states:

The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for a given pair of media and for a given colour of light.

Mathematically, Snell’s Law is expressed as:

sin(i) / sin(r) = constant = n2 / n1

Where:

• i is the angle of incidence.
• r is the angle of refraction.
• n1 is the refractive index of the first medium.
• n2 is the refractive index of the second medium.

The constant value is the refractive index of the second medium with respect to the first medium (n21 = n2/n1). If light travels from medium 1 to medium 2, then n1 sin i = n2 sin r.

Refractive Index (n)

The refractive index is a dimensionless quantity that describes how fast light propagates through a 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 indicates an optically denser medium where light travels slower, and bends more towards the normal when entering from a rarer medium. For example, water has a refractive index of approximately 1.33, and glass around 1.5.

Direction of Bending: Identification

The direction in which light bends depends on the optical densities of the media:

1. From Optically Rarer to Optically Denser Medium: Light slows down and bends towards the normal. (e.g., Air to Water)
2. From Optically Denser to Optically Rarer Medium: Light speeds up and bends away from the normal. (e.g., Water to Air)

Condition for No Refraction: When a ray of light is incident normally (perpendicularly) on the interface between two media (i.e., angle of incidence i = 0 degrees), it passes undeviated. In this case, the angle of refraction r = 0 degrees.

Quick Revision Points

• Refraction is the bending of light at the boundary of two media.
• Caused by a change in light’s speed in different media.
• First Law: Incident ray, refracted ray, normal are coplanar.
• Second Law (Snell’s Law): sin(i) / sin(r) = constant (ratio of refractive indices).
• Refractive index (n) = Speed of light in vacuum / Speed of light in medium.
• Rarer to denser: Bends towards normal.
• Denser to rarer: Bends away from normal.
• Normal incidence (i=0): No refraction, ray passes undeviated.

Extra Practice Questions

1. Define refraction of light.
2. State the two laws of refraction of light.
3. A ray of light traveling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why?
4. What is meant by the refractive index of a medium?
5. Under what condition does a ray of light pass undeviated when traveling from one medium to another?