Refraction of Light Class 10 Notes Maharashtra Board

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Std 10 Science Part 1 Chapter 6 Refraction of Light Notes Maharashtra Board

Class 10 Science Part 1 Chapter 6 Refraction of Light Notes Maharashtra Board

Refraction of light:

Definition: Light changes its direction when going from one transparent medium to another transparent medium. This is called the refraction of light.

Refraction of light passing through a glass slab:

Let us study the light ray entering a glass slab from air. Here AN is the incident ray and NB is the refracted ray.

  1. Incident ray and refracted ray at the point of incidence N are on the opposite sides of the normal to the surface of the slab at that point i.e. CD, and the three, incident ray, refracted ray and the normal, are in the same plane.
  2. For a given pair of media, here air and glass, the ratio of sin i to sin r is a constant. Here, i is the angle of incidence and r is the angle of refraction.

Refractive index:

  1. The change in the direction of a light ray while entering different media is different.
  2. It is related to the refractive index of the medium.
  3. The value of the refractive index is different for different media and also for light of different colours for the same medium.
  4. The refractive index of a medium with respect to vacuum is called its absolute refractive index.
  5. Refractive index depends on the velocity of light in the medium.

7. n is called the refractive index of the second medium with respect to the first medium.

8. The second law is also called Snell’s law.

9. A ray incident along the normal (i = 0) goes forward in the same direction (r = 0).

    Light ray going from medium 1 to medium 2:

    Similarly, the refractive index of medium 1 with respect to medium 2 is

    If the first medium is vacuum then the refractive index of medium 2 is called absolute
    refractive index and it is written as n.

    Refraction of light in different media:

    A) Light ray passes from a rarer medium to a denser a medium:

    When a light ray passes from a rarer medium to a denser a medium, it bends towards the
    normal.

    B) Light ray passes from a denser medium to a rarer medium:

    When a light ray passes from a denser medium to a rarer medium, it bends away from the
    normal.

    C) Light ray is incident normally at the boundary between two media:

    When a light ray is incident normally at the boundary between two media, it does not
    change its direction and hence does not get refracted.

    Twinkling of stars:

    A) Why do stars twinkle?

    1. Stars are very far away, so they look like tiny point sources of light.
    2. As starlight passes through Earth’s atmosphere, it moves through layers of air with
      different densities.
    3. The denser the air (closer to Earth), the higher the refractive index (bending power).
    4. Light bends towards the normal (towards denser medium), making the star appear
      slightly higher in the sky.
    5. Because the atmosphere is constantly moving and its temperature/density keeps
      changing, the light bends differently all the time.
    6. This constant change makes the star’s position and brightness seem to flicker →
      twinkling effect.

    B) Why don’t planets twinkle?

    1. Planets are much closer to Earth compared to stars.
    2. They appear as small disks or a group of point sources, not single tiny points.
    3. Atmospheric changes affect individual point sources, but when you combine all the
      light, the average position and brightness stay the same.
    4. Result: Planets do not twinkle.

    C) Why do we see the Sun before sunrise and after sunset?

    1. Sunrise: Even when the Sun is just below the horizon, its light bends (refracts)
      through the atmosphere and reaches our eyes.
    2. This makes the Sun appear above the horizon before it actually rises.
    3. Sunset: After the Sun has gone below the horizon, we can still see it for a short time
      because its rays bend through the atmosphere.
    4. So, both at sunrise and sunset, atmospheric refraction lets us see the Sun a little
      longer.

    Mirage:

    A) Hot surfaces make air layers rarer

    1. Near hot roads, deserts, or fires, the air close to the surface is hot and less dense
      (rarer).
    2. As you go higher, the air becomes cooler and denser, so the refractive index
      increases with height.

    B) Example 1: Mirage on hot roads or deserts

    1. Light from a distant object (like a tree) passes through layers of air with different
      refractive indices.
    2. The light bends (refracts) gradually away from the normal because it’s moving from
      denser to rarer layers near the ground.
    3. Eventually, the light rays bend so much they totally reflect upward, reaching your
      eyes.
    4. Your brain thinks the light is coming from the ground → you see an image below the
      object, like water on the road.
    5. This is called a mirage.

    C) Example 2: Objects near Holi fire look like they’re moving

    1. Above the flames, the hot air causes constant changes in the refractive index.
    2. Light from objects behind the fire bends in different directions as it passes through
      these layers.
    3. This bending makes the objects appear shaky or wavy, as if they are moving.

    Dispersion of light:

    Definition:

    The process of separation of light into its component colours while passing through a
    medium is called the dispersion of light.

    A) What do you see through a plastic scale or prism?

    When you look through the plastic scale (or prism) and turn it slowly, you see the
    light splitting into seven colours:
    Violet, Indigo, Blue, Green, Yellow, Orange, Red (remember: VIBGYOR).

    B) Why does light split into colours?

    1. Light is a type of electromagnetic radiation.
    2. Our eyes can detect wavelengths between 400 nm (violet) and 700 nm (red).
    3. Red light → longest wavelength (~700 nm).
    4. Violet light → shortest wavelength (~400 nm).

    C) How does speed change in a medium?

    1. In vacuum, all colours of light travel at the same speed.
    2. In a medium (like glass), the speed depends on the colour (frequency).
    3. So, refractive index (how much the light bends) is different for each colour.

    D) What happens when white light enters a prism?

    1. When white sunlight enters the glass prism, different colours bend by different amounts. Red bends the least, violet bends the most.
    2. As a result, the colours spread out and form a spectrum.

    E) Who first discovered this?

    Sir Isaac Newton was the first to use a glass prism to show the Sun’s spectrum.

    Partial and total internal reflection:

    A) What happens when light goes from denser to rarer medium?

    1. Example: glass to air, water to air.
    2. Part of the light reflects back into the denser medium → partial reflection.
    3. Part of the light refracts (bends) and passes into the rarer medium.

    B) How does light bend?

    1. When moving from denser to rarer, light bends away from the normal.
    2. This means: Angle of incidence i < Angle of refraction r.

    C) What happens as we increase the angle of incidence?

    1. As i increases, r also increases (follows Snell’s Law).
    2. For a particular i, r becomes exactly 90° → this ii is called the critical angle.

    D) What if the angle of incidence is larger than the critical angle?

    1. Light can no longer refract into the rarer medium.
    2. Instead, all the light reflects back into the denser medium.
    3. This is called total internal reflection (TIR).

    Rainbow production:

    A) What is a rainbow?

    1. A beautiful natural phenomenon seen mainly after rain.
    2. It happens because of the combined effects of: Dispersion (splitting of light), Refraction (bending of light), Total Internal Reflection (light bouncing inside droplets).

    B) What causes it?

    1. After rainfall, small water droplets remain in the air.
    2. These droplets act like tiny prisms.

    C) What happens to sunlight?

    Step 1: Refraction → Sunlight enters the droplet and bends.
    Step 2: Dispersion → Light splits into seven colours (VIBGYOR).
    Step 3: Total Internal Reflection → Inside the droplet, light reflects off the inner
    surface.
    Step 4: Refraction again → Light bends again as it exits the droplet.

    D) Final result

    1. All these processes combine to send the coloured light back to your eyes.
    2. You see the beautiful arc-shaped rainbow with seven colours.

    Solutions for the exercises, unsolved numerical problems, and solved examples will soon be available for download in convenient PDF format at very affordable prices. Our notes are carefully crafted, accurate, and aligned with board requirements to help you succeed.

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