Lenses
- A lens forms an image by refracting light.
- In a convex (converging) lens, parallel rays of light are brought to a focus at the principal focus. The distance from the lens to the principal focus is the focal length.
- A concave (diverging) lens spreads parallel rays out, so they appear to come from the principal focus.
- Images can be real (light rays actually meet; it can be shown on a screen) or virtual (rays only appear to come from it).
Magnification
- Magnification = image height ÷ object height. It has no units because both heights are in the same units.
- A convex lens used as a magnifying glass (object closer than the focal length) makes a virtual, upright, magnified image.
- A concave lens always forms a virtual, upright, diminished (smaller) image.
- In ray diagrams, a convex lens is drawn as a line with outward arrows and a concave lens with inward arrows.
Visible light and colour
- Each colour in the visible spectrum has its own narrow band of wavelength and frequency, from red (longest wavelength) to violet (shortest).
- The colour of an opaque object depends on which wavelengths are most strongly reflected; the others are absorbed.
- If all wavelengths are reflected equally, the object looks white. If all are absorbed, it looks black.
- Transparent and translucent objects transmit light. Colour filters absorb some wavelengths and transmit others: a red filter transmits only red light.
Black body radiation
- All bodies (objects), whatever their temperature, emit and absorb infrared radiation. The hotter the body, the more infrared radiation it emits in a given time.
- A perfect black body absorbs all the radiation that hits it, and is also the best possible emitter.
- A body at constant temperature absorbs radiation at the same rate as it emits it. The Earth's temperature depends on the balance of radiation absorbed and emitted.
Key terms
- Convex lens
- A converging lens that brings parallel rays to a focus.
- Concave lens
- A diverging lens that spreads parallel rays out.
- Principal focus
- The point where parallel rays meet (or appear to come from) after passing through a lens.
- Focal length
- The distance from the centre of a lens to the principal focus.
- Real image
- An image formed where light rays meet; it can be projected on a screen.
- Virtual image
- An image from which light rays only appear to come; it can't be projected.
- Magnification
- Image height ÷ object height.
- Colour filter
- A material that transmits some wavelengths of light and absorbs the rest.
- Black body
- An object that absorbs all the radiation falling on it; the best emitter.