Lenses and visible light

Physics · Waves. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A lens forms an image by refracting light. A convex lens brings parallel rays together at the principal focus. A concave lens spreads parallel rays out so that they seem to come from the principal focus. The distance from the lens to the principal focus is the focal length.

A real image is formed where rays actually meet, so it can be shown on a screen. A virtual image is where rays only appear to come from.

Each colour of light has its own narrow band of wavelength and frequency. An opaque object has the colour of the wavelengths it reflects most strongly. A colour filter transmits some wavelengths and absorbs the others.

Rules to remember

magnification = image height ÷ object height. It is a ratio, so it has no unit; use the same unit for both heights.
Convex lens, object beyond the principal focus: real, inverted image. Object inside the focal length: virtual, upright, magnified image.
Concave lens: the image is always virtual, upright and diminished.
Ray diagram: a ray parallel to the principal axis passes through the principal focus; a ray through the centre of the lens goes straight on.
Specular reflection: smooth surface, one direction. Diffuse reflection: rough surface, light scattered.
An object that reflects all wavelengths equally looks white; one that absorbs all wavelengths looks black.
Transparent: transmits light clearly. Translucent: transmits but scatters light. Opaque: transmits no light.

Common mistake

Students divide the wrong way round and get object height ÷ image height. A magnified image must give a magnification greater than 1, and a diminished image less than 1.

Worked example

A convex lens forms an image 1.5 cm tall of an object 6.0 cm tall. What is the magnification?

magnification = image height ÷ object height.
magnification = 1.5 ÷ 6.0 = 0.25.
The value is less than 1, so the image is smaller than the object (diminished).

Answer: 0.25 (no unit)

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