Sound, hearing, ultrasound and ultraviolet light

People hear sounds of about 20 to 20,000 vibrations per second. Vibration faster than 20,000 is called ultrasound; people cannot hear it, but bats and dolphins can and also make it. A bat finds obstacles in the dark by the return of its ultrasound signals. Using this idea, people made the echo sounder to measure the sea depth, and doctors use ultrasound to examine internal organs. Sunlight also has ultraviolet rays that we cannot see and that burn the skin. The colour of skin, hair and eyes depends on melanin: more melanin gives a darker colour. People whose ancestors lived in sunny regions have more melanin, which protects the skin from the sun. Different colours do not mean that one person is better than another: all people are equal. The bodies of albino people make little or no melanin, so they must be especially careful in the sun. Protection: light long-sleeved clothes, a hat, sunglasses, sun cream and staying in the shade around midday.