Diffraction and lensesIB MYP Sciences: Revision notes
Section 1
Diffraction of waves
Diffraction is the spreading out of waves when they pass through a gap or move past the edge of an obstacle. The wavelength, speed and frequency do not change.
- Gap about the same size as the wavelength: a lot of spreading, with semicircular waves.
- Gap much wider than the wavelength: little spreading, and the waves mostly carry straight on.
- A narrower gap gives more diffraction.
Sound has a long wavelength (about 1 m), so we hear it round corners. Light has a tiny wavelength, so we do not see round corners.
Diffraction does not change the wavelength. Only the direction and spread of the waves change.
Section 2
Converging and diverging lenses
A lens is a curved piece of transparent material that refracts light.
- A converging (convex) lens is thicker in the middle. It brings parallel rays together at the principal focus.
- A diverging (concave) lens is thinner in the middle. It spreads parallel rays apart, so they seem to come from a point behind the lens.
The focal length is the distance from the lens to the principal focus. A more curved lens bends light more and has a shorter focal length.
Section 3
Ray diagrams for lenses
Three rays are enough to find an image formed by a converging lens:
- A ray parallel to the axis is refracted through the principal focus.
- A ray through the centre of the lens carries straight on.
- A ray through the focus on the object side leaves parallel to the axis.
Where the rays meet gives the image. If the object is further than the focal length, the image is real and inverted. If the object is closer than the focal length, the rays spread out and the image is virtual, upright and magnified.
Section 4
Uses of lenses
- Magnifying glass: a converging lens with the object closer than the focal length gives a virtual, upright, magnified image.
- Camera: a converging lens forms a real, smaller, inverted image on a sensor or film.
- The eye: the lens refracts light to form a sharp image on the retina. A long-sighted person needs a converging lens in spectacles; a short-sighted person needs a diverging lens.
Section 5
Curved mirrors in outline
Curved mirrors reflect light using the law of reflection but change the direction of the rays.
- A concave mirror curves inwards and brings parallel rays together at a focus. It is used in shaving mirrors, torches and satellite dishes.
- A convex mirror curves outwards and spreads rays out, giving a wide field of view. It is used in shop security mirrors and car wing mirrors.
Must know
- Diffraction is the spreading of waves at a gap or edge; most when the gap is about one wavelength
- Converging lenses focus parallel rays; diverging lenses spread them
- Focal length is the distance from the lens to the principal focus
- A magnifying glass has the object closer than the focal length, giving a virtual, magnified image
- Concave mirrors converge rays; convex mirrors spread them
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Diffraction and lenses
- Waves in a harbour at Mombasa pass through a narrow gap in a sea wall into a sheltered bay. The gap is about 5 m wide, which is similar to the wavelength of the waves.Describe the shape of the waves after they pass through the 5 m gap and state what happens to their wavelength.2 marks
- A student uses a converging lens as a magnifying glass to read the small print on a medicine label. The lens has a focal length of 10 cm.Describe the image that the student sees when the label is in the correct position.2 marks
- A group of students tests the idea that the more curved a converging lens is, the shorter its focal length. They use two lenses made of the same glass and with the same diameter: lens A is strongly curved and lens B is gently curved. Each lens is used to focus the image of a distant window onto a white screen, and the distance from the lens to the screen is measured with a metre rule. Four readings are taken for each lens. For lens A the readings are 9.8 cm, 10.2 cm, 10.0 cm and 10.0 cm. For lens B they are 19.6 cm, 20.4 cm, 20.0 cm and 20.0 cm.Calculate the mean focal length of each lens and state whether the results support the students' idea.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).