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"Science in the Public Service" Forums and Lectures - August 2020

In light of the latest situation of the COVID-19, the arrangements for the talks are subject to change. Please stay tuned for the latest updates of our programmes on this webpage. A series of public forums and talks on the theme “Science in Everyday Life” will be held from August to November this year. Forum and talks in August are : (1) Talks: "Physics Spots Pneumonia, and other Medical Applications", "The Latest Development of Harnessing Renewable Energy in Drainage Services Department" and "Regional Air Pollution and LiDAR Application" at 2:30-4:45pm on 22 August 2020. (2) Forum: "Application of Innovation and Technology in Rainstorm and Landslip Warning Systems" and Talk: "Life Changing 5G Use Cases" at 2:30-4:45pm on 30 August 2020. The forum and talks in August will be live broadcast only. Please stay tuned on the Hong Kong Science Museum Facebook page https://www.facebook.com/hkscm. All forum and talks in August will be conducted in Cantonese.  Don't miss the chance to enrich your knowledge in science and to better understand the scientific work of various government departments!

Why does wet sand look darker than dry sand?

Water is colourless and transparent, but why is wet sand darker than dry sand? Is it because water absorbs more light than air? Not entirely correct. All else being equal, wet sand looks darker because not much light is coming out. For sand grains in water, the change in the direction of light is smaller than in air. On average, it takes a much longer path for light in wet sand to come out (left) than in dry sand (right). The longer the path, the greater the chance that light gets absorbed. Hence, wet sand looks darker than dry sand. (For more details, please click here to read the article written by Hong Kong Observatory) (Information provided by Hong Kong Observatory)

Myths about protection against UV radiation

Although most people are aware of the need to take protective actions against sunburn, there are some common misconceptions about UV radiation and the ways of protection: Myth 1: Darker sunglasses offer more protection from UV radiationThe most important thing to look for in sunglasses is how much UV radiation they filter out.  It should be noted that there is no relationship between the colour of sunglasses and their UV filtering action.  When one wears sunglasses the pupil widens as there is less light reaching the eye.  If the sunglasses have poor UV protection, the amount of UV radiation getting into the eyes may even be greater than not wearing sunglasses at all.  For adequate protection, one should wear sunglasses that are able to block at least 98% of the UV radiation. Myth 2: You can't get sunburn on a cloudy dayYou do get sunburned on a cloudy day, that is of course, if you are engaged in outdoor activities but not properly protected against ultraviolet radiation (UV).  It is true that on a cloudy day, you won't get as much exposed to UV from direct sunlight as when it is a clear sunny day.  However, sunlight, including UV, are scattered by gases in the atmosphere, as well as by clouds, dust, haze and even fog. Up to 80% of solar UV radiation can penetrate thin cloud cover.  There are also occasions when broken clouds enhance UV radiation by reflection from their sides.Therefore, in case of doubt, it is advisable to check the latest UV index through radio, television, the Observatory's website and Dial-a-Weather system (1878200). Myth 3: You can't get sunburn while in the waterWater offers only minimal protection from UV radiation.  At half a metre under water, the UV radiation level is still 40% as intense as at the surface.  Also, the part of body above water is additionally exposed to ultraviolet rays reflected from the water surface. Myth 4: Sunscreen lotion protects me so I can sunbathe much longerSunscreen lotion should not be used to increase sun exposure time but to increase protection during unavoidable exposure. The protection provided by sunscreen lotion depends critically on their correct application.  For more about sunscreen lotion and sunburn protection, please check out "Sunburn and SPF". Myth 5: If you take regular breaks during sunbathing you won't get sunburnUV radiation exposure is cumulative.  The total health damage you get will be the sum of the effect of individual exposure.  Therefore, to protect yourself the objective is to reduce exposure to UV radiation as far as practicable. This could decrease the chance of skin cancer. Myth 6: If you don't feel the hot rays of the sun you won't get sunburnSunburn is caused by UV radiation which cannot be felt.  The heating effect is caused by the sun's infrared radiation and not by UV radiation.  Therefore, even if you don't feel the warmth, you may also get sunburn. Human exposure to UV radiation may result in acute and chronic health effects on the skin, eye and immune system. So please be aware to take protective actions against sunburn. (For more details, please click here to read the article written by Hong Kong Observatory) (Information provided by Hong Kong Observatory)

The physics of gurgling

What causes the bubbling sound when drinking from a bottle?When drinking from an inverted bottle, a vacuum starts to appear at the top as water flows out of the bottle. Because of the air pressure outside, air forces its way through the neck of the bottle and bubbles up. This is followed by more water escaping, and more air bubbles moving up. So on and so forth. The glug-glug is caused by these two alternating processes.Does water flow faster at the beginning or near the end?Water flows faster at the beginning because this is when the pressure is highest.Does water flow faster when the bottle is in an inverted position (i.e. upside down) or when it is tilted?(The experiment can be carried out quite easily in a kitchen or bathroom. The result accords with our experience.)Water flows faster when the bottle is tilted, i.e. at an angle. This avoids the gurgling, i.e. air bubbles coming up through the liquid, which obstructs the passage of water.What is the fastest way to pour out water?The fastest way is to pour water at an angle and with a swirl. To create a swirl, move the bottle in small circles before pouring. This way, water moves to the side of the bottle and no gurgling occurs, allowing air to freely enter through the centre (For more details, please click here to read the article written by Hong Kong Observatory) (Information provided by Hong Kong Observatory)