Science Class 12 - Optics Notes

Optics

Welcome to the chapter on Optics for Class 12. In this chapter, you will learn about the behavior of light, its properties, and how it interacts with different materials. By the end of this chapter, you will understand the principles of reflection, refraction, lenses, mirrors, and optical instruments, and be able to solve related numerical and conceptual problems.

Key Concepts

• Light: A form of energy that enables us to see objects.
• Reflection: The bouncing back of light from a surface.
• Refraction: The bending of light as it passes from one medium to another.
• Lenses: Transparent objects that refract light to form images.
• Mirrors: Smooth surfaces that reflect light to form images.
• Optical Instruments: Devices that use lenses and/or mirrors to enhance vision (e.g., microscope, telescope, human eye).

Reflection of Light

Reflection occurs when light strikes a surface and bounces back. The laws of reflection state:

• The angle of incidence is equal to the angle of reflection.
• The incident ray, reflected ray, and normal all lie in the same plane.

Example: Plane mirrors form virtual, erect images of the same size as the object.

Refraction of Light

Refraction is the bending of light as it passes from one medium to another due to a change in speed. The laws of refraction (Snell's Law) are:

• The incident ray, refracted ray, and normal all lie in the same plane.
• The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant (n₁sinθ₁ = n₂sinθ₂).

Example: A straw appears bent in a glass of water due to refraction.

Lenses and Mirrors

• Convex lens: Converges light rays; used in magnifying glasses.
• Concave lens: Diverges light rays; used in spectacles for myopia.
• Concave mirror: Can form real or virtual images; used in shaving mirrors.
• Convex mirror: Always forms virtual, diminished images; used as rear-view mirrors.

Lens formula: 1/f = 1/v - 1/u
Mirror formula: 1/f = 1/v + 1/u

Optical Instruments

• Human Eye: Natural optical instrument; forms real, inverted images on the retina.
• Microscope: Used to see tiny objects; uses two lenses for high magnification.
• Telescope: Used to see distant objects; uses lenses or mirrors to gather and focus light.

Practice Questions

1. State the laws of reflection of light.
2. What is Snell's Law? Write its mathematical form.
3. A convex lens has a focal length of 10 cm. Where will the image form if an object is placed 20 cm from the lens?
4. Why does a coin appear raised when placed in water?
5. Name two uses of convex mirrors.

Challenge Yourself

• Draw ray diagrams for image formation by a concave mirror for different object positions.
• Explain the working of a compound microscope with a neat diagram.

Did You Know?

• The speed of light in vacuum is about 3 × 10⁸ m/s!
• The human eye can adjust its focal length to focus on near and far objects (accommodation).

Glossary

• Refraction: Bending of light as it passes from one medium to another.
• Focal length (f): The distance from the lens or mirror to its focus.
• Magnification: The ratio of the height of the image to the height of the object.
• Optical axis: The straight line passing through the center of a lens or mirror.

Answers to Practice Questions

1. 1. The angle of incidence equals the angle of reflection. 2. The incident ray, reflected ray, and normal lie in the same plane.
2. Snell's Law: n₁sinθ₁ = n₂sinθ₂
3. Using lens formula: 1/f = 1/v - 1/u ⇒ 1/10 = 1/v - 1/(-20) ⇒ 1/v = 1/10 - 1/20 = (2-1)/20 = 1/20 ⇒ v = 20 cm. The image forms 20 cm on the other side of the lens.
4. Due to refraction, light rays bend at the water surface, making the coin appear raised.
5. Used as rear-view mirrors in vehicles and as security mirrors in shops.

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Mastering optics helps you understand the science behind vision, cameras, and many modern technologies!