Waves, light and sound, Year 8: more practice

Three tiny bones in the middle ear

HintSmall linked parts sit between the eardrum and the inner ear.

The whySound waves shake the eardrum, the tiny bones pass the vibration on, and the inner ear turns it into signals that travel to the brain along a nerve.

Its frequency is above 20,000 Hz, the top of the human hearing range

HintCompare 30,000 with the upper limit of what our ears detect.

The whyHumans hear roughly 20 Hz to 20,000 Hz, and the top of the range falls with age. Dogs can hear well above 20,000 Hz.

Light travels in straight lines, so it cannot bend round the corner to your eyes

HintThink about the route the rays take between a friend and you.

The whyLight only reaches your eyes if there is a clear straight-line path from the object to you.

The shadow gets bigger

HintRays spread out from the lamp in straight lines. Which rays does the object block?

The whyClose to the object, the lamp sends a wider fan of rays at it, so the object blocks rays reaching a larger area of the wall.

70

HintThe normal is at 90° to the surface.

The whyThe angle of incidence is measured from the normal: 90° − 20° = 70°. The angle of reflection equals it, so it is 70°.

50

HintThe normal is at 90° to the surface. Find the reflected ray's angle to the normal first.

The whyThe angle of reflection is 40°, and the normal is at 90° to the surface, so the reflected ray is 90 − 40 = 50° from the mirror.

6

HintThe image is as far behind the mirror as the object is in front.

The whyThe image is 3 m behind the mirror, so the distance between object and image is 3 + 3 = 6 m.

Towards the normal

HintGlass slows light down.

The whyLight slows down in glass, so its path bends towards the normal. Leaving the block into air it speeds up and bends away from the normal.

Light from the bottom bends as it leaves the water, so it seems to come from a higher point

HintYour brain assumes rays always reach your eyes in a straight line.

The whyThe rays bend away from the normal as they leave the water into air. Traced straight back, they seem to start higher than the real bottom.

Fatter (more curved), so it bends light more

HintClose objects send rays in at a steep angle that needs bending harder.

The whyMuscles squeeze the lens fatter to focus on near objects and let it relax thinner for distant ones, keeping the image sharp on the retina.