Reflection of light
When light falls on a polished surface such as a mirror, it bounces back. This is reflection.
Some terms:
Incident ray: the ray of light falling on the surface.
Reflected ray: the ray that bounces back.
Normal: a line drawn at 90° to the surface at the point where the incident ray strikes.
Angle of incidence (∠i): the angle between the incident ray and the normal.
Angle of reflection (∠r): the angle between the reflected ray and the normal.
The laws of reflection
The angle of incidence equals the angle of reflection (∠i = ∠r).
The incident ray, the reflected ray and the normal all lie in the same plane.
Both angles are measured from the normal, not from the mirror. A ray striking the mirror at 30° to its surface has an angle of incidence of 90° − 30° = 60°.
A ray falling along the normal (∠i = 0°) is reflected straight back along the same path.
Regular and diffused reflection
Regular reflection: a smooth, polished surface (mirror, still water) reflects parallel rays in one direction, forming a clear image.
Diffused (irregular) reflection: a rough surface (paper, a wall, cardboard) reflects parallel rays in many directions. No image forms, but we can see the object from every side.
The laws of reflection still hold for each tiny part of a rough surface. The rays scatter only because those parts face different directions.
We see objects that don't give out their own light (illuminated objects) because they reflect light into our eyes. Objects that make their own light are luminous.
Images in a plane mirror
The image formed by a plane mirror is:
virtual (cannot be caught on a screen) and erect (upright),
the same size as the object,
as far behind the mirror as the object is in front,
laterally inverted: left and right are swapped.
Multiple images
Two mirrors placed at an angle form several images. The number of images is
n = (360° ÷ angle between the mirrors) − 1
when 360 divided by the angle is a whole number. At 90°, there are 360/90 − 1 = 3 images. At 60°, there are 5. Two parallel mirrors form, in principle, infinitely many images.
A kaleidoscope uses three mirror strips at 60° to each other, with bits of coloured glass between them, to make beautiful repeating patterns. A periscope uses two parallel plane mirrors at 45° to see over obstacles; submarines and soldiers in trenches use them.
Dispersion: sunlight is a mixture
When a beam of sunlight passes through a glass prism, it splits into a band of seven colours: violet, indigo, blue, green, yellow, orange, red (VIBGYOR). This splitting is called dispersion. A rainbow forms when tiny raindrops act like prisms and disperse sunlight.
The human eye
Part | Function |
|---|---|
Cornea | transparent front part; light enters through it |
Iris | coloured part; controls the size of the pupil |
Pupil | opening that lets light in; it narrows in bright light and widens in dim light |
Lens | focuses light onto the retina |
Retina | screen at the back of the eye where the image forms; has light-sensitive cells: rods (dim light) and cones (bright light and colour) |
Optic nerve | carries messages to the brain |
The point where the optic nerve leaves the retina has no light-sensitive cells, so it is called the blind spot.
The image on the retina lasts about 1/16 of a second after the object is gone (persistence of vision). Movies use this: still pictures shown faster than 16 per second appear to move.
For comfortable reading, the least distance of distinct vision for a normal eye is about 25 cm.
Caring for the eyes
Read in good light and keep a book about 25 cm away.
Never look directly at the Sun or at very bright light.
Don't rub your eyes; wash them with clean water if dust enters.
Eat foods rich in vitamin A (carrots, green leafy vegetables, milk, papaya). Its lack causes night blindness.
Visually impaired people can read using Braille, a system of raised dots invented by Louis Braille. Each character is made of up to 6 dots in a cell of 2 columns and 3 rows.
Summary
∠i = ∠r, and the incident ray, reflected ray and normal lie in one plane.
Smooth surfaces give regular reflection; rough surfaces give diffused reflection.
Two mirrors at angle θ give 360/θ − 1 images.
White light disperses into seven colours; the retina forms the image in the eye.