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Asked by anon-252499 on 30 Apr 2020.
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Malgorzata Drwila answered on 30 Apr 2020:
A very good question! As you correctly point out, when light passes through a lens, it bends and changes direction. We call this refraction. Refraction happens because the light travels at a different speed in the lens than it does in the air. The amount by which the light bends depends on the change in the speed at which light travels in the two materials (the air and the lens).
Lenses are clever because they are designed to be thicker in some areas and thinner in others thereby allowing us to focus the light at a specific location.
Shortsighted people (like me) have eyes which don’t focus light at the correct point at the back of our eye making far away things look blurry. The lenses in glasses are used to focus the light at the right point allowing us to see!
Another interesting point is that refraction doesn’t only occur for light, but for all kinds of waves. Seismic waves (those we feel in earthquakes) for example also change direction when they pass through rocks under the ground which have different properties. This helps us to understand what’s happening deep beneath our feet in the heart of Planet Earth.
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anon answered on 30 Apr 2020:
Refraction. This is valid for any wave passing a boundary between two different media (materials). Water waves (different water depth = different media), gravity waves (like after an earthquake)!
The word “refract” literally means “to break up” – because light of different wavelengths (=colours, for visible light) gets deflected by a different angle. -
Lisa Hollands answered on 30 Apr 2020:
This is called refraction. By going through the lens material, the light starts to travel at a different speed and changes direction of bends from its original path. This makes glass lenses really useful for helping to correct people’s eyesight by bending the light to be able to focus when the lens in the eye or eyeball shape is not correct
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Susan Cartwright answered on 30 Apr 2020:
I don’t think I have much to add to the other posts. The process of bending is called refraction (that’s what happens within a material, as opposed to reflection which is what happens when your light bounces off the surface of a material). The amount by which a given material bends light is determined by the material’s refractive index, which is 1 for no bending and >1 for materials like glass or water. An interesting point is that refractive index is wavelength dependent, so typically different colours will be bent by different amounts: this is what you see in a prism, for example. It also happens in lenses, causing an annoying effect called chromatic aberration whereby your focused image from the lens has coloured fringes round it, because blue and red light don’t focus at the same point. Modern compound lenses in binoculars and telescopes are made of more than one lens to reduce this effectL different types of glass have different refractive indices, so by combining lenses made from different types of glass you can nearly cancel out chromatic aberration (such compound lenses are called achromatic for this reason).
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James Smallcombe answered on 1 May 2020:
This effect, called refraction, happens when light (or any wave) crosses between two different materials (such as a glass lens and the air). Because light moves at different speeds in the two materials when it crosses at an angle the side of the light wave nearest the crossing changes speed first and this causes the wave to turn. You can see this in water waves as well where there is a sudden change in depth.
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