Light Energy
ICSE Class 8 · Physics · Chapter 5
What this chapter covers
- Refraction of light
- Examples of refraction in daily life
- Curved mirrors: concave and convex
- Terms used for spherical mirrors
- Rules for making ray diagrams
- Real and virtual images
- Ray diagrams for real images formed by a concave mirror
- Uses of curved mirrors
- Dispersion of white light
Key definitions
Refraction is the bending of light when it passes at a slant from one transparent medium into another.
Dispersion is the splitting of white light into its constituent colours when it passes through a prism or another transparent medium.
Key points
- Refraction is the bending of light when it passes at a slant from one transparent medium into another; it happens because light changes speed.
- Light going into a medium where it is slower bends towards the normal; light going into a faster medium bends away from it. A ray along the normal does not bend.
- A bent-looking pencil in water, a pool that looks shallow, a mirage, and advanced sunrise and delayed sunset are all caused by refraction.
- A spherical mirror is part of a hollow sphere. A concave mirror reflects from its hollow side and converges light; a convex mirror reflects from its bulging side and diverges light.
- Pole (P), centre of curvature (C), radius of curvature (R), principal axis, principal focus (F) and focal length (f) describe a spherical mirror; f = R/2.
- Ray diagrams use four rays: parallel to the axis, through F, through C, and to the pole. Two rays are enough to locate an image.
- A real image is formed where rays actually meet, can be caught on a screen and is inverted; a virtual image is formed where rays only appear to meet and is erect.
- A concave mirror forms a real, inverted image of an object placed beyond F; the image grows and moves away as the object moves from far away towards F.
- Concave mirrors are used in shaving mirrors, headlights, torches and solar cookers; convex mirrors are used as rear-view mirrors and at sharp road bends.
- Dispersion is the splitting of white light into seven colours (VIBGYOR) by a prism; violet bends the most and red the least. A rainbow is dispersion by raindrops.
Questions answered
Define refraction. When does a ray of light pass from one medium to another without bending?
Refraction is the bending of light when it passes at a slant from one transparent medium into another. A ray that strikes the surface along the normal, at right angles to it, passes straight through without bending.
A ray of light passes from air into glass. Does it bend towards or away from the normal? Give a reason.
It bends towards the normal, because light slows down when it goes from air into glass.
Why does a swimming pool look shallower than it really is?
Light from the bottom of the pool bends away from the normal as it leaves the water. Your eye traces the light back in a straight line, so the bottom appears raised and the pool looks shallower.
Explain why you can see the Sun a little before it rises above the horizon.
Sunlight bends as it passes through the layers of the atmosphere, which become denser close to the ground. The bent light reaches your eye while the Sun is still just below the horizon. This is called advanced sunrise.
State two differences between a concave mirror and a convex mirror.
A concave mirror has a reflecting surface curved inwards, while a convex mirror has one that bulges outwards. A concave mirror brings parallel rays together at a point, while a convex mirror spreads them apart.
Define principal axis and focal length for a spherical mirror.
The principal axis is the straight line passing through the pole and the centre of curvature of a spherical mirror. The focal length is the distance between the pole and the principal focus.
The radius of curvature of a concave mirror is 40 cm. What is its focal length?
f = (R)/(2) = (40)/(2) = 20 cm.
A ray passes through the centre of curvature of a concave mirror. What happens to it after reflection, and why?
It is reflected back along the same path. A ray through C meets the mirror along the normal, so the angle of incidence is zero and so is the angle of reflection.