Key Learning Outcomes
By the end of this lesson, readers will:
- ✔️ Understand the Laws of Reflection and how light behaves when it strikes a plane or curved mirror.
- ✔️ Differentiate between real and virtual images formed by concave and convex mirrors.
- ✔️ Learn the Laws of Refraction and explain how light bends when passing through different mediums.
- ✔️ Analyze the working of lenses (concave and convex) and their applications in optics.
- ✔️ Apply Snell’s Law to calculate the refractive index of different materials.
- ✔️ Understand Total Internal Reflection (TIR) and its practical applications like mirages and optical fibers.
- ✔️ Calculate focal length and magnification using mirror and lens formulas.
- ✔️ Determine the power of a lens and understand its significance in optical instruments.
Click On The Name To Go To A Specific Topic:
Introduction:
Light is a form of energy that enables us to see objects. It travels in a straight line and exhibits two key behaviors:
- Reflection of Light – The bouncing back of light from a surface.
- Refraction of Light – The bending of light when it moves from one medium to another.
Reflection of Light
Reflection is the phenomenon where light rays bounce back from a smooth or polished surface.
Laws of Reflection:
- The angle of incidence (i) is equal to the angle of reflection (r). (∠i = ∠r)
- The incident ray, the reflected ray, and the normal to the surface all lie on the same plane.
Types of Reflection:
1. Regular Reflection – From a smooth surface, producing a clear image.
2. Irregular (Diffuse) Reflection – From a rough surface, scattering light in different directions.
Spherical Mirrors:
Spherical mirrors are curved mirrors, of two types:
- Concave Mirror – Curved inward (like the inside of a spoon).
- Convex Mirror – Curved outward (like the back of a spoon).
Important Terms Related to Spherical Mirrors:
- Pole (P) – Center of the mirror’s surface.
- Center of Curvature (C) – Center of the sphere from which the mirror is cut.
- Principal Axis – The line passing through C and P.
- Focus (F) – The point where parallel rays converge (concave) or appear to diverge (convex).
- Focal Length (f) – Distance between F and P.
Mirror Formula:
where,
- f = focal length
- u = object distance
- v = image distance
Refraction of Light
Refraction is the bending of light when it passes from one transparent medium to another due to a change in speed.
Laws of Refraction:
- The incident ray, the refracted ray, and the normal all lie in the same plane.
- Snell’s Law: The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant.
where n is the refractive index of the medium.
Refractive Index (n):
- Higher n value → More bending of light.
- Lower n value → Less bending of light.
Refraction Through a Glass Slab:
- Light bends toward the normal when entering a denser medium.
- Light bends away from the normal when entering a rarer medium.
- The emergent ray is parallel to the incident ray but slightly displaced.
Lens Formula for Refraction in Lenses:
where,
- f = focal length
- u = object distance
- v = image distance
Power of a Lens:
Unit: Diopter (D)
- Convex lens → Positive Power
- Concave lens → Negative Power
Important Summary Points
✔️ Reflection: Light bounces off a surface following the laws of reflection.
✔️ Refraction: Light bends when it enters a new medium due to a change in speed.
✔️ Spherical Mirrors: Concave mirrors converge light, while convex mirrors diverge light.
✔️ Lenses: Convex lenses converge rays, while concave lenses diverge them.
✔️ Refractive Index: Determines how much light bends in a medium.
Interesting Facts
💡 Light travels at 3 × 10⁸ m/s in vacuum, the fastest speed in the universe.
💡 The Sun’s light takes about 8 minutes to reach Earth.
💡 Refraction causes rainbows and mirages!
Activity (Exercise):
A. Fill in the Blanks:
- The angle of incidence is always equal to the __________ in reflection.
- A convex mirror always forms a __________ image.
- The bending of light when it passes from air to water is called __________.
B. Short Answer Questions:
- What is lateral displacement?
- Why do stars twinkle at night?
- How does a concave mirror differ from a convex mirror?
C. Numerical Problems:
- A concave mirror has a focal length of 20 cm. An object is placed at 30 cm. Find the position and nature of the image.
- A convex lens has a focal length of 10 cm. Find its power.
Quiz:
Coming Soon…
Supplementary Materials:
Provide downloadable materials for learners to review:
- – PDF Guide: “Coming Soon”
- – Cheat Sheet: “Coming Soon”
- – Video Source: “JNG ACADEMY“
- – Articles: “Blog Page“
FAQs:
1. What is the difference between regular and diffused reflection?
Regular Reflection occurs on a smooth surface, where parallel light rays reflect in a specific direction, forming a clear image (e.g., plane mirror).
Diffused Reflection occurs on a rough surface, where parallel light rays scatter in different directions, not forming a clear image (e.g., paper surface).
Diffused Reflection occurs on a rough surface, where parallel light rays scatter in different directions, not forming a clear image (e.g., paper surface).
2. What is the law of reflection?
The laws of reflection state that:
a) The incident ray, the reflected ray, and the normal to the surface of reflection all lie in the same plane.
b) The angle of incidence is always equal to the angle of reflection.
a) The incident ray, the reflected ray, and the normal to the surface of reflection all lie in the same plane.
b) The angle of incidence is always equal to the angle of reflection.
3. What are the laws of refraction?
The laws of refraction state that:
a) The incident ray, refracted ray, and normal all lie in the same plane.
b) The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant, known as Snell’s Law:
n=sin(i) /sin(r)
where n is the refractive index.
a) The incident ray, refracted ray, and normal all lie in the same plane.
b) The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant, known as Snell’s Law:
n=sin(i) /sin(r)
where n is the refractive index.
4. What is the refractive index?
The refractive index (n) of a medium is the ratio of the speed of light in a vacuum to the speed of light in that medium.
n=c/v
where:
c = speed of light in vacuum
v = speed of light in the medium
n=c/v
where:
c = speed of light in vacuum
v = speed of light in the medium
5. What is total internal reflection?
When a light ray travels from a denser medium to a rarer medium at an angle greater than the critical angle, it reflects back into the denser medium instead of refracting. This is called Total Internal Reflection (TIR). Example: Mirage in deserts.
6. What is the difference between concave and convex mirrors?
Concave Mirror: A mirror with a reflecting surface curved inward, used in torches, headlights, and shaving mirrors.
Convex Mirror: A mirror with a reflecting surface curved outward, used in rear-view mirrors of vehicles.
Convex Mirror: A mirror with a reflecting surface curved outward, used in rear-view mirrors of vehicles.
7. What are the uses of concave and convex lenses?
Convex Lens: Used in magnifying glasses, cameras, and human eyes to converge light.
Concave Lens: Used in spectacles for myopia (nearsightedness) and in laser devices to diverge light.
Concave Lens: Used in spectacles for myopia (nearsightedness) and in laser devices to diverge light.
8. What is the power of a lens?
The power (P) of a lens is the reciprocal of its focal length (f) in meters:
P=1/f (in diopters, D)
a) Convex lens has positive power.
b) Concave lens has negative power.
P=1/f (in diopters, D)
a) Convex lens has positive power.
b) Concave lens has negative power.
9. Why does a pencil appear bent in water?
A pencil appears bent in water due to refraction—light changes speed and direction as it moves from water to air.