Revision: Refraction of Light Science and Technology 1 SSC (English Medium) 10th Standard Maharashtra State Board

Light rays that are parallel to the principal axis of a concave mirror converge at a specific point on its principal axis after reflecting from the mirror. This point is known as the principal focus of the concave mirror.

Refracted light is the part of light enters into the other medium and travels in a straight path but in a direction different from its initial direction and is called the refracted light.

The change in the direction of the path of light when it passes from one transparent medium to another transparent medium is called refraction. The refraction of light is essentially a surface phenomenon.

When travelling obliquely from one medium to another, the direction of propagation of light in the second medium changes. This phenomenon is known as refraction of light.

OR

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

OR

The bending of the light ray from its path in passing from one medium to the other medium is called 'refraction' of light.

The refractive index of second medium with respect to the first medium is defined as the ratio of the sine of the angle of incidence in the first medium to the sine of the angle of refraction in the second medium.

The light rays coming from a distant object appear to be coming from the image of the object inside the ground. This is called a mirage.

The phenomenon of splitting of white light by a prism into its constituent colours is known as dispersion.

OR

The splitting of light into its component colours is called dispersion.

OR

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

The phenomenon of the splitting of white light by a prism into its constituent colours is known as dispersion of light.

When a beam of white light or composite light is refracted through any transparent media such as glass or water, it is split into its component colours. This phenomenon is called ‘dispersion of light’.

On passing white light through a prism, the band of colours seen on a screen is called the spectrum.

or

The band of the coloured components of a light beam is called its spectrum.

When light enters a rarer medium from a denser medium, it gets partially reflected i.e. part of the light gets reflected and comes back into the denser medium as per laws of reflection. This is called partial reflection.

For a particular value of i, the value of r becomes equal to 90°. This value of i is called the critical angle.

For angles of incidence larger than the critical angle, the angle of refraction is larger than 90°. Such rays return to the denser medium. Thus, all the light gets reflected back into the dense medium. This is called total internal reflection.

The refractive index of medium 2 w.r.to medium 1:

\[n_{21} = \frac{\text{Speed of light in medium 1}}{\text{Speed of light in medium 2}} = \frac{v_1}{v_2}\]

The refractive index of medium 1 w.r.to medium 2:

\[n_{12}=\frac{\text{Speed of light in medium 2}}{\text{Speed of light in medium 1}}=\frac{v_{2}}{v_{1}}\]

The refractive index of the medium n_m  is given by:

\[n_{m} = \frac{\text{Speed of light in air}}{\text{Speed of light in the medium}} = \frac{c}{v}\]

OR

\[\frac {\text{sin i}}{\text{sin r}}\] = constant = n

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

• The incident ray, the refracted ray and the normal at the point of incidence, all lie in the same plane.
• The ratio of the sine of the angle of incidence i to the sine of the angle of refraction r is constant for the pair of given media. i.e., mathematically

\[\frac {\text{sin i}}{\text{sin r}}\] = constant = n

Where:

• i = angle of incidence
• r = angle of refraction
• n = refractive index of the second medium with respect to the first

This ratio is constant for a given pair of media and is known as the refractive index.

Note: If a light ray is incident along the normal (i = 0^∘), it passes undeviated into the second medium (r = 0^∘).

• Mirage: Looks like water on a hot road; caused by light bending in hot, less dense air.
• Shaking Objects Near Fire: Hot air near the fire bends light unevenly, making objects look like they are moving.
• Star Twinkling: Stars twinkle because their light bends through moving air layers in the atmosphere.
• Planets Don’t Twinkle: Planets are closer and appear bigger, so changes in light cancel out.
• Early Sunrise & Late Sunset: We see the Sun even before it rises and after it sets because of light bending in the atmosphere.

• Dispersion is the splitting of white light into seven colours (VIBGYOR) when it passes through a prism or similar transparent medium.
• Human eyes can detect light with wavelengths ranging from 400 nm (violet) to 700 nm (red).
• Different colours travel at different speeds in a medium like glass, so each colour has a different refractive index.
• Violet light bends the most, and red light bends the least, as it passes through a prism, producing a spectrum.
• A rainbow is formed due to dispersion, refraction, and internal reflection of sunlight by raindrops acting as tiny prisms.

• Light going from a denser to a rarer medium partly reflects and partly bends away from the normal.
• If we keep increasing the angle of incidence, the refracted ray becomes 90° at a certain point — this is the critical angle.
• Beyond the critical angle, all light reflects back into the denser medium — this is total internal reflection.
• A rainbow forms due to refraction, dispersion, and total internal reflection of sunlight in raindrops.
• Snell’s law explains how the angle of refraction depends on the angle of incidence and the refractive index.