Light, Reflection and the Human Eye: Class 8 Study Guide
Light bounces off surfaces by fixed rules, and the eye uses light to form pictures.
The laws of reflection say that the angle of incidence equals the angle of reflection, and that the incident ray, the reflected ray and the normal (a line at right angles to the mirror) all lie in the same plane. The image in a plane mirror is virtual (it cannot be shown on a screen), upright, the same size as the object, as far behind the mirror as the object is in front, and left and right are reversed.
White light splits into seven colours when it passes through a prism. This is dispersion. A rainbow forms when sunlight is dispersed by raindrops.
The human eye
| Part | Job |
|---|---|
| Cornea | Clear front layer that lets light in and bends it |
| Iris | Coloured ring that controls the size of the pupil |
| Pupil | Opening that lets light in |
| Lens | Focuses light on the retina |
| Retina | Layer of light-sensitive cells |
| Optic nerve | Sends signals to the brain |
Seeing and caring for the eyes
Light enters the eye through the cornea and pupil, and the lens focuses it on the retina. The retina sends signals along the optic nerve, and the brain makes the picture. Read in good light, avoid screens held very close, never rub the eyes with dirty hands, eat foods rich in vitamin A and have regular eye tests. Some people wear glasses to correct their vision.
Cornea and pupil.
On the retina.
Optic nerve.
The picture.
Reading without sight
Braille, invented by Louis Braille, is a system of raised dots arranged in cells of six, which people read by touch. Every letter has its own pattern, so blind and visually impaired people can read and write.
Worked examples and common mistakes
Example: A ray hits a mirror at 40ยฐ to the normal. The reflected ray is also at 40ยฐ. Example: If you stand 2 m from a plane mirror, your image appears 2 m behind it, so it is 4 m from you. Common mistakes: measuring the angle from the mirror's surface rather than from the normal, and thinking that the iris is the opening (the pupil is).
Check your understanding: model answers
1. The iris. 2. As far behind the mirror as the object is in front. 3. Six.
โจ Key Takeaways
- Angle of incidence = angle of reflection.
- A plane mirror image is virtual, upright and laterally reversed.
- A prism splits white light into seven colours.
- The retina receives the image.
๐ฌ Questions for Discussion & Reflection
- Which part of the eye controls the size of the pupil?
- Where does the image form in a plane mirror?
- How many dots are in a Braille cell?
Print a science observation log โ