Ever wondered why people say to wear white clothes in summer? Here’s why.
When an object appears any particular color, it is because it reflects that color of light and absorbs all other colors. For example, Green leaves appear green as they reflect green light and absorb all other colors.
Similarly, for an object to appear white, it is has to reflect white color but white color can be obtained only by the mixture of all the colors. Therefore, all colors are reflected for white. Whereas an object appears black when no light is reflected or when no light reaches the eye. Therefore all the colors are absorbed.
White reflects all colors of light. An object that looks red reflects only red light and absorbs all the other colors. Something that appears black absorbs every color and reflects none. leaf appears green because it reflects the green light and absorbs the rest of the colors.
Now we need to understand the link between the light and heat.
When light hits an object, the light energy excites electrons in the object and the atoms start vibrating. Some atoms vibrate sufficiently such that, their vibrational energy is almost equal to the electronic energy from the sun, then they are in resonance (both vibrate at the same frequency) with solar energy. These atoms turn from electronically excited to virbrationally excited. Which makes the whole atom move. This motion is seen as HEAT.
So on a hot day, if you happen to wear a black shirt, all the light will be absorbed and then turned into heat and makes you feel very hot.
Thermal expansion – All materials are made of atoms. At any temperature above absolute zero (-273 degrees Celsius) the atoms start moving. In a solid state they will be vibrating in fixed positions, in a liquid they will be moving past each other and in a gas they will be rushing past each other at very high speeds, the atoms in all these states also collide with other atoms. When a material is heated, the kinetic energy (energy possessed due to motion) of that material increases and it’s atoms and molecules move and vibrate more. This also increases the collision of atoms, so they will be pushing and barging into each other. This means each atom will cover more space and hence, the material will expand. When it is cold the kinetic energy decreases, so the atoms take up less space and the material contracts.
That is why builders have to be very careful while building bridges and rails.
They have to leave little space between the rails, for expansion and contraction to take place.
Thermal expansion depends on bond energy. If the bond energy is strong then more energy is required to break the bond and therefore, the expansion will be less.
this is why the thermal expansion of gases is greater then liquids which is greater than solids. because bond energy is solid is the highest followed by liquids then gases.
nikhil erigila 