Waves, light and sound, Year 8: more practice

Three tiny bones in the middle ear

提示Small linked parts sit between the eardrum and the inner ear.

为什么Sound 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

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

为什么Humans 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

提示Think about the route the rays take between a friend and you.

为什么Light only reaches your eyes if there is a clear straight-line path from the object to you.

The shadow gets bigger

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

为什么Close 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

提示The normal is at 90° to the surface.

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

50

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

为什么The 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

提示The image is as far behind the mirror as the object is in front.

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

Towards the normal

提示Glass slows light down.

为什么Light 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

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

为什么The 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

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

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