Light helps us see things.
An object sends light into our eyes. Usually, the object reflects light from a source such as the Sun, a bulb, or a candle.
In complete darkness, our eyes cannot see because there is no light entering them.
Light usually travels in straight paths.
Think of light as tiny arrows moving straight ahead. This is why an object can make a sharp shadow when it blocks light.
A narrow path of light is called a ray.
Reflection means light bouncing back after hitting a surface.
A mirror reflects a lot of light, so we can see our face in it.
The angle at which light arrives is equal to the angle at which it leaves.
The incoming ray, the reflected ray, and the normal all lie in the same flat plane.
The normal is an imaginary line drawn straight up from the surface.
A plane mirror is a flat mirror.
The image in a plane mirror is:
Upright
The same size as the object
The same distance behind the mirror as the object is in front
Laterally inverted, meaning left and right appear switched
Virtual, meaning it cannot be caught on a screen
A spherical mirror is a curved mirror that looks like part of a ball.
There are two types:
Concave mirror
Convex mirror
A concave mirror curves inward, like the inside of a spoon.
It can bring light rays together, so it is called a converging mirror.
A concave mirror can make images that are:
Real or virtual
Upright or upside down
Bigger, smaller, or the same size
Examples of uses:
Shaving mirrors
Dentist mirrors
Torches
Searchlights
Vehicle headlights
Solar furnaces
A convex mirror curves outward, like the back of a spoon.
It spreads light rays apart, so its reflected rays appear to come from behind the mirror.
A convex mirror always makes an image that is:
Virtual
Upright
Smaller than the object
It gives a wide view, so it is used as a vehicle’s rear-view mirror.
The pole is the middle point of the mirror’s surface.
This is the centre of the imaginary sphere from which the mirror was made.
This is the distance from the pole to the centre of curvature.
This is the imaginary straight line passing through the pole and centre of curvature.
This is the point where parallel rays meet, or seem to come from, after reflection.
This is the distance from the pole to the focus.
For a small-aperture spherical mirror:
[ R = 2f ]
The image depends on where the object is placed.
Object position | Image result |
|---|---|
Very far away | Tiny, real, upside down |
Beyond C | Smaller, real, upside down |
At C | Same size, real, upside down |
Between C and F | Bigger, real, upside down |
At F | Image forms very far away |
Between F and P | Bigger, virtual, upright |
A concave mirror is special because it can make many different kinds of images.
A convex mirror always makes an image that is:
Upright
Virtual
Smaller
If the object is very far away, the image is extremely tiny.
If the object comes closer, the image becomes a little bigger, but it remains smaller than the object.
Ray diagrams help us find where an image forms.
Useful rules:
A ray parallel to the principal axis reflects through the focus of a concave mirror.
A ray passing through the focus reflects parallel to the principal axis.
A ray passing through the centre of curvature reflects back along the same path.
A ray hitting the pole follows the ordinary rules of reflection.
Usually, two rays are enough to find the image.
Scientists use signs to show directions.
The pole is treated as the starting point.
Distances to the left are negative.
Distances to the right are positive.
Heights above the principal axis are positive.
Heights below the principal axis are negative.
For an object placed in front of a mirror, the object distance is usually negative.
The mirror formula connects object distance, image distance, and focal length:
[ \frac{1}{f}=\frac{1}{v}+\frac{1}{u} ]
Here:
(u) = object distance
(v) = image distance
(f) = focal length
Use the correct signs when solving problems.
Magnification tells us how large the image is compared with the object.
[ m=\frac{h'}{h}=-\frac{v}{u} ]
Here:
(h') = image height
(h) = object height
Meaning:
(m>1): image is bigger
(m<1): image is smaller
Negative magnification: image is upside down
Positive magnification: image is upright
Refraction means light bending when it enters a different material.
For example, a pencil partly inside water may look bent.
This happens because light travels at different speeds in different materials.
A material in which light travels faster is called optically rarer.
A material in which light travels slower is called optically denser.
Examples:
Air is optically rarer than glass.
Glass is optically denser than air.
When light moves:
From rarer to denser: it bends toward the normal.
From denser to rarer: it bends away from the normal.
The incoming ray, bent ray, and normal are in the same plane.
For the same two materials and the same colour of light:
[ \frac{\sin i}{\sin r}=\text{constant} ]
This is called Snell’s law.
Here:
(i) = angle of incidence
(r) = angle of refraction
When light enters a rectangular glass slab:
It bends toward the normal while entering glass.
It bends away from the normal while leaving glass.
The outgoing ray is parallel to the incoming ray.
The outgoing ray is shifted sideways.
It is like walking onto a slower surface at an angle, then walking back onto the faster surface.
The refractive index tells us how much a material slows down light and bends it.
[ n=\frac{\text{speed of light in air or vacuum}}{\text{speed of light in the material}} ]
A larger refractive index usually means light travels more slowly in that material.
For example, diamond has a high refractive index, so it bends light strongly.
A lens is a transparent object with at least one curved surface.
There are two main types:
Convex lens
Concave lens
A convex lens is thicker in the middle and thinner at the edges.
It brings parallel light rays together, so it is called a converging lens.
It is used in:
Magnifying glasses
Cameras
Microscopes
Telescopes
Spectacles
A concave lens is thinner in the middle and thicker at the edges.
It spreads light rays apart, so it is called a diverging lens.
A concave lens always forms an image that is:
Virtual
Upright
Smaller
It is often used in spectacles for correcting short-sightedness.
The middle point of the lens.
A ray passing through the optical centre travels almost straight without bending.
An imaginary straight line through the optical centre and the lens’s centres of curvature.
For a convex lens, parallel rays meet at the focus.
For a concave lens, parallel rays spread out as if they came from the focus.
The distance from the optical centre to the focus.
Object position | Image result |
|---|---|
Very far away | Tiny, real, upside down |
Beyond 2F | Smaller, real, upside down |
At 2F | Same size, real, upside down |
Between F and 2F | Bigger, real, upside down |
At F | Image forms very far away |
Between F and O | Bigger, virtual, upright |
A convex lens can make either real or virtual images.
A concave lens always produces a:
Virtual image
Upright image
Smaller image
The image forms between the lens and its focus, on the same side as the object.
Useful rules:
A ray parallel to the principal axis passes through the focus after a convex lens.
A ray parallel to the principal axis appears to come from the focus after a concave lens.
A ray passing through the focus of a convex lens leaves parallel to the principal axis.
A ray through the optical centre continues nearly straight.
Distances are measured from the optical centre.
A convex lens has positive focal length.
A concave lens has negative focal length.
Distances to the left are negative.
Distances to the right are positive.
The lens formula is:
[ \frac{1}{f}=\frac{1}{v}-\frac{1}{u} ]
Here:
(u) = object distance
(v) = image distance
(f) = focal length
[ m=\frac{h'}{h}=\frac{v}{u} ]
It tells us how large the image is compared with the object.
Positive magnification means the image is upright.
Negative magnification means the image is upside down.
A value greater than 1 means the image is enlarged.
A value less than 1 means the image is smaller.
Power tells us how strongly a lens bends light.
[ P=\frac{1}{f} ]
The focal length must be measured in metres.
The unit of power is the dioptre, written as D.
Convex lens: positive power
Concave lens: negative power
Example:
A lens with focal length (0.5) m has:
[ P=\frac{1}{0.5}=+2D ]
When lenses touch each other, their powers can be added:
[ P=P_1+P_2+P_3 ]
For example:
[ +2D + +1D = +3D ]
Mirrors bounce light, lenses bend light, and both can make images.