Light & Optics Revision Notes
Reflection: Even vs Scattering + Law of Reflection
Even (Specular) Reflection
Happens when light hits smooth, shiny surfaces like mirrors or glass.
The rays bounce off all in the same direction, creating a clear image.
Example: Seeing your reflection in a mirror.
Scattering (Diffuse Reflection)
Happens on rough surfaces like paper or wood.
Light rays bounce off in many directions, resulting in no clear image or a fuzzy one.
Example: Not being able to see your reflection on a wall.
Law of Reflection
The angle of incidence equals the angle of reflection.
The angle where light hits the surface is the same as the angle it bounces off.
Angles are measured from the normal, an imaginary line at 90^\circ (right angle) to the surface, for accuracy.
Refraction: Bending of Light + Law of Refraction
What is Refraction?
Refraction occurs when light bends due to a change in speed as it moves between different materials.
Slowing Down (Bending Towards the Normal)
When light enters a denser material (e.g., air to glass or water), it slows down and bends toward the normal.
Speeding Up (Bending Away From the Normal)
When light enters a less dense material (e.g., glass to air), it speeds up and bends away from the normal.
Law of Refraction (Snell’s Law)
The amount of bending is proportional to the change in speed; the more light slows down or speeds up, the more it bends.
How Images Are Formed – Ray Diagrams
To determine where an image forms, draw two key light rays:
One ray goes straight, parallel to the axis, and then refracts or reflects.
The second ray goes through the focus or the center of the lens/mirror.
Difference Between Real Image and Virtual Image
Feature | Real Image | Virtual Image |
|---|---|---|
Orientation | Inverted | Erect |
Lens/Mirror Types | Formed by a convex lens or a concave mirror | Formed by concave, convex, or plane mirrors |
Screen Projection | Can be formed on a screen | Appears to be on the lens or mirror itself; cannot be projected on a screen |
Light Ray Intersection | Formed due to the actual intersection of light rays | Formed due to the imaginary intersection of light rays |
Types of Images
Real Image: Can be projected onto a screen.
Virtual Image: Visible in a mirror but cannot be projected onto a screen.
Two Types of Lenses and Their Uses
Convex Lens (Converging)
Bulges out in the middle.
Causes light rays to come together (converge).
Focuses light to a point.
Uses: magnifying glasses, eyes, microscopes, cameras.
Concave Lens (Diverging)
Curves inward like a cave.
Causes light rays to spread out (diverge).
Uses: glasses for short-sightedness, door peepholes.
How Light Changes at Boundaries
When light moves between two different materials (an interface):
If it slows down, it bends toward the normal.
If it speeds up, it bends away from the normal.
This is another way of explaining refraction.
How Lenses Work (Ray Model Explanation)
Lenses bend light by making it change speed.
The ray model demonstrates how this works:
Light travels in straight lines until it interacts with something.
Convex lenses bend light inward, converging at one point.
Concave lenses make light spread outward.
The material and shape of the lens determine the amount of bending.
Splitting White Light With a Prism
How It Works:
A prism is a triangle-shaped glass block.
When white light enters, it refracts and spreads into colors.
Each color bends differently—red bends the least, and violet bends the most.
Colors in Order (Visible Spectrum): Red, Orange, Yellow, Green, Blue, Indigo, Violet (ROYGBIV).
Why Objects Look Colored
An object's color is determined by the color of light it reflects and the colors it absorbs.
A red apple looks red because it reflects red light and absorbs all other colors.
If you shine blue light on a red apple, it may appear black because there is no red light to reflect.