Light, mirrors and lensesIB MYP Physics: Revision notes
Section 1
Light, rays and shadows
Light is a transverse wave and travels at 3 × 10⁸ m/s in a vacuum (much faster than sound). It travels in straight lines, which we draw as rays with arrows.
Because light travels in straight lines, an opaque object blocks it and makes a shadow. A shadow is an area that light cannot reach. A shadow gets bigger when the object moves closer to the light source.
Section 2
The pinhole camera
A pinhole camera is a light-proof box with a tiny hole at the front and a screen at the back. Rays from each point of an object pass through the hole in straight lines, so the image on the screen is real and inverted (upside down).
- Moving the object closer makes the image larger.
- A smaller hole makes the image sharper but dimmer.
- A larger hole lets in more light but makes the image blurred.
Section 3
Converging and diverging lenses
A converging (convex) lens is thicker in the middle. It bends parallel rays inwards to meet at a point called the principal focus (or focus). A diverging (concave) lens is thinner in the middle and spreads parallel rays outwards.
- The principal axis is the line through the centre of the lens at right angles to it.
- The focal length is the distance from the centre of the lens to the principal focus.
- A thicker, more curved lens has a shorter focal length because it bends light more strongly.
You do not need a lens formula; you only draw ray diagrams.
Section 4
Ray diagrams: real and virtual images
For a converging lens, draw two rays from the top of the object:
- a ray parallel to the principal axis that bends to pass through the focus;
- a ray through the centre of the lens that goes straight on.
Where they cross is where the top of the image is.
- Object further than twice the focal length: image is real, inverted and smaller (camera, eye).
- Object between one and two focal lengths: image is real, inverted and larger.
- Object closer than the focal length: the rays spread out, and the image is virtual, upright and magnified (magnifying glass).
A real image can be caught on a screen. A virtual image cannot; you see it by looking into the lens. A diverging lens always gives a virtual, upright, smaller image.
Do not say a virtual image is 'not there'. It can be seen, but the rays only appear to come from it, so it cannot be put on a screen.
Section 5
Uses of lenses and the eye
- Magnifying glass: a converging lens used with the object closer than the focal length.
- Camera: a converging lens makes a real, inverted, smaller image on a sensor or film.
- Eye: the lens focuses light onto the retina, making a real, inverted, smaller image.
- Short sight: distant objects are focused in front of the retina. A diverging lens corrects it.
- Long sight: near objects would be focused behind the retina. A converging lens corrects it.
Must know
- Light is a transverse wave; 3 × 10⁸ m/s in a vacuum; travels in straight lines.
- Pinhole camera: real, inverted image; closer object gives a bigger image.
- Converging lens brings parallel rays to the focus; diverging lens spreads them.
- Draw ray diagrams using a ray parallel to the axis and a ray through the centre.
- Real images go on a screen; virtual images do not. Magnifying glass: virtual, upright, magnified.
- Short sight: diverging lens. Long sight: converging lens.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Light, mirrors and lenses
- A group of students build a pinhole camera from a cardboard tube. One end is closed with foil that has a tiny hole in the middle, and the other end is covered with tracing paper that acts as a screen. They point the hole at a lit candle in a dark room and see an image of the flame on the screen.The students move the camera closer to the candle. State what happens to the size of the image and explain why.2 marks
- Amira is short-sighted. Light from distant objects is focused in front of her retina, so distant objects look blurred, and she wears glasses to correct this. Her brother Idris is long-sighted, and he uses a magnifying glass to read the small print on medicine packets.State the type of lens in Idris's magnifying glass and describe the image he sees.2 marks
- A student investigates how the shape of a converging lens affects its focal length. She holds each lens so that light from a distant window passes through it, then moves a white card behind the lens until a sharp image of the window forms. She measures the distance from the lens to the card. Lens P is thin and only slightly curved, lens Q is thicker, and lens R is the thickest and most strongly curved. Her results are 30 cm for P, 15 cm for Q and 6 cm for R.Identify the independent variable, the dependent variable and one control variable in this investigation.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).