AQA Physics (8463) 4.6.2.5 · Model
Lenses
The same lens can show a page one way and a window another. What decides which image you get, and can you call it before the rays are drawn?
Triple Science · Higher tier
Start here
One magnifying glass, a page and a window.
Hold a magnifying glass just above a page and the words look bigger and the right way up. Now hold the same lens at arm’s length and look through it at a window across the room.
If you had to guess, do you see the window the right way up, or upside down?
A lens forms an image by refracting light. A convex (converging) lens brings parallel rays together at the principal focus, F; the distance from the lens to F is the focal length. A concave (diverging) lens spreads parallel rays out, as if they came from F on the near side. To find an image, follow two rays from the top of the object: one parallel to the axis, which a convex lens bends through F, and one through the centre of the lens, which goes straight on. Where they meet is the top of the image. If the rays really meet, the image is real and a screen can catch it. If they only seem to come from a point, it is virtual.
The lens bench · move the object, call the image
Convex lens, object 30 cm away
0 of 7 set-ups
Real or virtual?
How big?
Which way up?
Your turn · complete the ray diagram
Object beyond 2F
Step 1 of 4
Ray 1 leaves the top of the object parallel to the axis. Tap a point it passes through after the lens.
Compare the lenses
Convex, concave, or both?
Each statement is about the image a single lens forms of a real object.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Lenses in spectacles · AQA Biology 4.5.2.3
In myopia (short sight) the eyeball is too long or the eye lens too strongly curved, so light from distant objects focuses in front of the retina. In hyperopia (long sight) the eyeball is too short or the lens not curved enough, so light from near objects would focus behind it.
Your call
PredictThis short-sighted eye focuses light from a distant object in front of the retina. Which spectacle lens moves the focus back onto the retina?
Equations
magnification = image height ÷ object height
image height = magnification × object height
Units first
Both heights in the same unit, mm or cm: 1 cm = 10 mm. Magnification is a ratio, so it has no unit. Bigger than 1: magnified. Less than 1: diminished.
Command words in this lesson
- Describe the image
- Real or virtual, upright or inverted, magnified or diminished: one mark each.
- Complete the ray diagram
- Two rays from the top of the object, with arrows. Dashed lines for rays traced back.
- Calculate
- Show the working. A magnification has no unit.
- Compare
- Say how both lenses differ, in the same sentence.
Key fact
Convex lens: object beyond F, real and inverted image; object inside F, virtual, upright and magnified; object at F, no image. Concave lens: always virtual, upright and diminished.
Key note · AQA 4.6.2.5 (8463)
Lenses
Photograph this card. Tomorrow, cover it and say each line first.
Previous and next
Connects to
Ask MrBadmus
Not sure where the image goes, or what kind it is?
The bench treats each lens as a thin lens with a 10 cm focal length, and draws each ray bending once, at the centre line of the lens, as AQA's ray diagrams do; a real lens bends light at both surfaces. Horizontal and vertical scales differ so tall images fit, which keeps every ray straight and every crossing point true. The eye diagram is simplified.