Lenses and Visible LightAQA GCSE Physics: Revision notes
Section 1
How Does a Lens Form an Image?
A lens forms an image by refracting light as it enters and leaves the lens.
A convex (converging) lens is curved outward and brings parallel rays of light together to a single point called the principal focus. The focal length is the distance from the centre of the lens to the principal focus.
Section 2
What Kinds of Image Can a Lens Produce?
- A convex lens can produce either a real image (formed where light rays actually meet, and can be projected onto a screen) or a virtual image (where light rays only appear to come from, and cannot be projected onto a screen), depending on the object's distance from the lens.
- A concave (diverging) lens spreads parallel rays of light apart, and always produces a virtual image.
Ray diagrams can be used to show the similarities and differences between the images formed by convex and concave lenses.
A concave lens can never form a real image — if a question describes a real, projectable image, the lens must be convex.
Section 3
How Do You Calculate Magnification?
Magnification compares the size of the image formed by a lens with the size of the original object:
magnification = image height ÷ object height
Magnification has no units, since it is a ratio of two lengths. A magnification greater than 1 means the image is larger than the object; less than 1 means it is smaller.
An object 2 cm tall forms an image 8 cm tall. Magnification = 8 ÷ 2 = 4.
Section 4
Why Do Objects Appear to Have Different Colours?
The colour of an opaque object is determined by which wavelengths of visible light it reflects — the other wavelengths are absorbed.
- A red object reflects red wavelengths of light and absorbs the others.
- A colour filter only transmits certain wavelengths of light and absorbs the rest, so light passing through it changes colour.
- Specular reflection occurs off a smooth surface (a clear, single reflected image, e.g. a mirror); diffuse reflection occurs off a rough surface (light scattered in many directions, e.g. paper).
Section 5
What Is the Difference Between Transparent and Translucent Objects?
- A transparent object allows almost all light to pass straight through it, so objects can be seen clearly behind it (e.g. clear glass).
- A translucent object allows some light through but scatters it, so objects behind it appear blurred (e.g. frosted glass).
Viewing an object through a colour filter changes the apparent colour of the object, because only certain wavelengths reach the eye.
Must Know
- A lens forms an image by refracting light; a convex lens brings parallel rays to a principal focus at the focal length.
- Convex lenses can form real or virtual images; concave lenses always form virtual images.
- Magnification = image height ÷ object height (no units).
- The colour of an opaque object is determined by the wavelengths of light it reflects; the rest are absorbed.
- Specular reflection (smooth surface) gives a clear image; diffuse reflection (rough surface) scatters light.
- Transparent objects let light through clearly; translucent objects scatter light so objects behind appear blurred.
That's the notes covered.
Carry on to the next subtopic.