Hash Tag - Youth.gov.hk
Skip to main content


Search Result: 9

Why are spectacles or camera lens so difficult to clean?

Why are spectacles or camera lens so difficult to clean? It seems to get worse when an ordinary cloth is used, which tends to produce mult-coloured marks.When an ordinary cloth is used to rub grease off the glass, the grease becomes an additional coating with varying thickness. This produces multi-coloured reflections, because light is made up of different colours (i.e. different wavelengths). The 'rainbow' effect is similar to what we see when looking at a wet road with an oil film on it.What is the proper way to clean the glasses?Use the special cloth that comes with spectacles or camera lens. It is made of thin fibres that are good at picking up grease, rather than spreading it.A wet method involves washing with soap or a little liquid detergent. Then rinse, and leave it to dry or dry with a clean linen. You can also go to an optical shop for such a service. (For more details, please click here to read the article written by Hong Kong Observatory) (Information provided by Hong Kong Observatory)

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)

How does the weather affect running?

How does the weather during the day and at night affect running?  How does temperature affect relative humidity?  What things do we need to pay attention to before and after running?  Karen Cheng and Dr. Lobo H.T. Louie will explain these in this episode of "Cool Met Stuff".(The programme is broadcasted in Cantonese) (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)

Why does lightning always come before thunder?

“The God of Thunder was empowered by the Jade Emperor to punish bad people on the Earth. “ “Once, the God of Thunder mistakenly killed a kind-hearted woman.  The Jade Emperor, having looked into the matter, raised the woman from death and named her the Goddess of Lightning.  He also dictated that whenever the God of Thunder stroke the Earth, he must let the Goddess of Lightning release light first to distinguish the right from wrong and prevent injustice. “ The above is just a Chinese legend. In nature, a lightning flash and the associated thunder occur at almost the same time in a thunderstorm. A person on the ground sees the lightning flash before hearing the thunder because light at a speed of around 300,000,000 meters per second travels much faster than sound which moves at 340 meters per second.  If one is 1,000 meters away from the thunderstorm, he/she would see the flash almost instantly after lightning occurs as it takes just a few microseconds, while the thunder arrives only after about 3 seconds (1,000 meters divided by 340 meters per second). As a rule of thumb, by counting the seconds between the flash and the thunder and dividing the number by 3, you can estimate your distance from the thunderstorm in kilometers.  For example, if you hear the thunder 9 seconds after seeing the flash, the thunderstorm should be about 3 kilometers away from you.  If you see a flash and hear a thunder clap almost simultaneously, the storm must be very near you. Seek shelter immediately. (For more details, please click here to read the article written by Hong Kong Observatory) (Information provided by Hong Kong Observatory)

Experimental Officer

Mr Ken Wong, an Experimental Officer of the Hong Kong Observatory, talks about his career as an Experimental Officer.An Experimental Officer is mainly deployed on weather forecasting, data processing, radioactivity, hydrometeorological, physical oceanographic, seismological, and time services duties.An Experimental Officer may be required to attend a professional training course in meteorology during his/her probationary period. He/She may be required to work outdoors, shifts or outside normal office hours. Organisation chartOfficial recruitment page

Why do stars twinkle?

Since no astronaut while traveling in space has reported seeing stars twinkling, the effect must be atmospheric, i.e. due to air. However, it is not exactly correct to say that it is due to turbulence in the air.Mere turbulence in the air is just what we call wind. Wind does not make stars twinkle, because light travels at a great speed --- over 1 billion km/h.What distorts the light coming from a star is temperature variations in the air. As you probably know already, air temperature varies a great deal. It typically decreases by 6.5 degrees Celsius for every kilometre you go up, and this accords with the experience that it feels cooler up in the mountains. Also, on a hot day, you may notice the shimmering waves (thermals) that come off a heated road and make a distant car appear wavy.But exactly how do temperature variations cause twinkling?When light enters a transparent medium, such as air, it generally changes direction, i.e. it is scattered. By how much it changes direction, i.e. bent, however, depends on the temperature.Now any star, except the sun, is so far away that practically, it is sending only a single ray of light towards us. As that ray enters the atmosphere, it is scattered differently as it passes through air of different temperatures. When it is scattered away from us, the star seems to disappear for a moment. When it is scattered into our eyes, it seems to reappear, resulting in a twinkle.For more details, please click here to read the article written by Hong Kong Observatory. (Information provided by Hong Kong Observatory)

Why "a few degrees lower over the New Territories" often appears in the weather forecasts?

The phrase "a few degrees lower over the New Territories" often appears in the weather forecasts during autumn and winter, in order to remind the public of the temperature difference between different regions. This episode of "Cool Met Stuff" will briefly explain the cause of this phenomenon in the form of a playlet.    (Information provided by Hong Kong Observatory)