Lights

 

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1. What is Light? 

Answer: Light is a form of energy that enables us to see objects. It travels in a straight line and does not require a material medium (it can travel in vacuum).

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2. State the two laws of reflection of light.

Answer:

1. The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.

2. The angle of incidence (i) is equal to the angle of reflection (r):

$$i = r$$

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3. What is the difference between regular and diffused reflection?

Answer:

Regular Reflection	Diffused Reflection
Occurs on smooth surfaces.	Occurs on rough surfaces.
Forms clear images.	Forms blurred or no image.
Example: Plane mirror.	Example: Paper, wall.

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4. What is the mirror formula?

Answer: For spherical mirrors:

$$\frac{1}{f}=\frac{1}{v}+\frac{1}{\mathrm{u}}$$

where

• f = focal length

• u = object distance

• v = image distance

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5. Define refraction of light.

Answer: Refraction is the bending of light when it travels from one medium to another due to change in its speed.

Example: Pencil in water appears bent.

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6. State Snell’s Law of Refraction.

Answer: Snell’s Law states that:

$$\frac{\sin i}{\sin r}=n_{\mathrm{21}}$$

where

• i = angle of incidence

• r = angle of refraction

• $n_{21}$ = refractive index of second medium w.r.t first

  
  

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7. What is refractive index?

Answer: Refractive Index (n) is the ratio of the speed of light in vacuum to the speed of light in the medium:

$$n = \frac{c}{v}$$

More refractive index → light bends more.

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8. Why does a coin placed in water appear raised?

Answer: Due to refraction, light bends away from the normal when it comes from water to air, making the coin appear at a higher position (virtual image). 

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9. What is total internal reflection (TIR)?

Answer: TIR occurs when

1. Light travels from a denser to rarer medium, and

2. Angle of incidence is greater than critical angle.

Example: Sparkling of diamonds, optical fibres.

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10. Define power of a lens and its unit.

Answer: Power of a lens refers to its ability to converge or diverge light.

$$P=\frac{1}{f(\text{in metres})}$$

Unit: Dioptre (D)

Positive power → convex lens
Negative power → concave lens

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11. Why does a convex lens form real and inverted images?

Answer: Because light rays passing through a convex lens converge and meet at a point (real focus), forming a real image on a screen.

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12. Write any two applications of refraction.

Answer:

1. Formation of rainbow

2. Working of magnifying glass

3. Stars appear twinkling

4. Lenses in spectacles

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13. Why does the sky appear blue?

Answer: Due to scattering of light. Blue light is scattered more than red because its wavelength is shorter. 

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14. Why do stars appear twinkling but planets do not?

Answer: Stars are far away, so the atmosphere causes variations in refraction → twinkling.
Planets appear larger → effect averages out → no twinkling.

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15. Explain why the sun appears red during sunrise/sunset.

Answer: Blue light is scattered away; red light (long wavelength) reaches our eyes → sun looks red.

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

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

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