Lights- Refraction and Reflection

 

🔵 Introduction to Light

Light is a form of energy that helps us to see the world.
In Class 10 Physics, the first chapter explains two important phenomena of light:

• Reflection of Light

• Refraction of Light

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🔶 1. Reflection of Light

Reflection means bouncing back of light from a smooth surface.

Laws of Reflection

• The incident ray, reflected ray and the normal all lie in the same plane.
• Angle of incidence (i) = Angle of reflection (r)

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🔵 Important Question – 1

Q. A ray of light strikes a plane mirror at an angle of incidence 30°. What will be the angle of reflection?

Answer: Angle of reflection = Angle of incidence
                                               = 30°

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🔶 2. Refraction of Light

Refraction is the bending of light when it passes from one medium to another (air → water, air → glass etc.).

➡ Light bends towards the normal when it enters a denser medium
➡ Light bends away from the normal when it enters a rarer medium

          

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Snell’s Law of Refraction

$$\frac{\sin i}{\sin r} = \text{constant} = n$$

Where n = refractive index of the second medium w.r.t first medium.

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🔵 Important Question – 2

Q. A ray of light enters glass from air at 30°. If the refractive index of glass is 1.5, find the angle of refraction.

Solution: Using Snell’s law:

$$\frac{\sin i}{\sin r} = n$$
$$\frac{\sin 30°}{\sin r} = 1.5$$
$$\sin r = \frac{1/2}{1.5} = \frac{1}{3}$$ $$r = \sin^{-1}\left(\frac{1}{3}\right)$$

                   Thus, Angle of refraction ≈ 19.5°

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🔶 3. Refraction Through Lenses

There are two types of lenses:

Convex Lens (Converging)

– Makes light rays meet at a point
– Used in magnifying glasses

        

 

Concave Lens (Diverging)

– Spreads out light rays
– Used in spectacles for myopia

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🔵 Important Question – 3

Q. What is the focal length of a lens?

Answer: The distance between the optic centre of a lens and its focus is called focal length.

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🔵 Important Question – 4

Q. A convex lens forms a real image of an object 40 cm away. If the image is formed at 80 cm on the other side of the lens, find the focal length.

Solution:  Using lens formula:

$$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$$

                 Given: u = –40 cm,  v = +80 cm

$$\frac{1}{f} = \frac{1}{80} - \left(\frac{1}{-40}\right)$$
$$\frac{1}{f} = \frac{1}{80} + \frac{1}{40} = \frac{3}{80}$$ $$f = \frac{80}{3} = 26.7\text{ cm}$$