## Plane mirror

Definition of the plane mirror

A plane mirror is a mirror that has a flat surface, where one of its surfaces is coated with reflective metal so that the surface of the plane mirror can reflect more than 95 percent of the light that strikes it. If you shine a flashlight toward a plane mirror surface, the plane mirror reflects the light so that your eyes glare if the mirror reflects the flashlight to your eyes. Conversely, if you stand in front of a mirror surface during the day, your face reflects sunlight toward the surface of the mirror, and at the same time, the mirror reflects light back toward you. If you stand in front of a plane mirror surface at night, then your face reflects the light of the electric light

towards the plane mirror surface, and at the same time, the plane mirror reflects the light of the electric light back towards you.

## Specular reflection and diffuse reflection

Article about Specular reflection and diffuse reflection

If you stand in front of a plane mirror, you can see the image of your face reflected by the plane mirror. If you stand on the edge of a plane mirror, can you still see your face behind the plane mirror? You can investigate this further by shining a flashlight on a flat mirror surface. When the flashlight is directed perpendicular to the surface of the plane mirror, the flashlight reflected by the plane mirror can be seen. But if you stand on the edge of a plane mirror, then light the flashlight towards the center of the flat mirror, then you cannot see the reflection of the flashlight!

Different things happen when you observe an object during the day. Things like books, stones, or tables can be seen during the day because they reflect sunlight. These objects, like a mirror in the previous problem. When an object is in front of a plane mirror, the object reflects sunlight toward the surface of the plane mirror. Sunlight reflected by an object towards the surface of the plane mirror is then reflected by a plane mirror towards the eye so that we can see the shadow of the object behind the plane mirror. Different things happen to books, stones, or tables. During the day, sunlight is emitted to the book, and the book reflects sunlight into the eyes, so we can see the book. At night, the light is emitted to the book, and the book reflects the light of the lamp towards the eye, so we can see the book.

## Law of reflection of light

Article about Law of reflection of light

Definition of reflection

Reflection can be interpreted as an event in which the direction of motion of an object changes due to the object hits a barrier. If you throw a ball on the wall, the ball reflected because the ball hits the wall.

Reflection of light

When light is emitted by the sun or other light sources such as electric lights, light moves from the light source in all directions. When light strikes a barrier such as a book, wall or mirror, light is reflected by that barrier. The direction of the light reflection after it strikes a barrier object is different from the direction of motion of the light before it strikes the barrier. It can be said that the reflection of light is an event in which light strikes a barrier so that the direction of light changes; the direction of movement of light after strikes a barrier object is different from the direction of motion of light before strikes a barrier.

## Wavefront Ray Beam of light

Article Wavefront Ray Beam of light

Definition of wavefront

If you throw a stone into a pool of water, circular water waves moving away from the center of the circle. In the water waves, there are crests and troughs. Wave peaks or waves valleys that move away from the center of the circle can be said as a wavefront.

Two adjacent wavefronts are two parts of the wave that has the same phase and are separated by one wavelength. Wave peaks or wave valleys that have the same phase and are separated by one or more wavelengths are referred to as wavefront. In addition to water waves, sound waves or light waves also have a wavefront. In contrast to water waves that move only in a plane or area, sound waves and light waves move in space.

## Poiseuilles equation

Poiseuille equation was discovered by Jean Louis Marie Poiseuille (1799-1869). As explained, each fluid can be considered as an ideal fluid. The ideal fluid does not have viscosity. If we assume an ideal fluid flows in a pipe, each part of the fluid moves at the same rate (v). Unlike the ideal fluid, the real fluid we encounter in everyday life has viscosity. Because it has a viscosity, then when flowing in a pipe, for example, the rate of each part of the fluid varies. The fluid layer that is in the middle moves faster (deep v), on the contrary, the fluid layer attached to the pipe does not move (v = 0). So from the middle to the edge of the pipe, every part of the fluid moves at different rates. To facilitate your understanding, observe the picture below.

## Viscosity

Fluid, both liquid and gas substances of different types, have different viscosity levels. Viscosity is a friction between the molecules that make up a fluid. So, the molecules make up fluid friction when the fluid flows. In liquids, the viscosity is caused by cohesion forces (pulling forces between similar molecules). Whereas in gas substances, the viscosity is caused by collisions between molecules.

Liquid fluid is easier to flow, for example, water. Conversely, thick fluid is more difficult to flow, for example, lubricant. You can prove by pouring water and lubricant on an inclined surface. Water flows faster than oil. The level of viscosity of fluid also depends on temperature. The higher the temperature of the liquid, the less viscous the liquid is. For example, when a mother fry fish in the kitchen, cooking oil which initially thickens becomes more liquid when heated. Conversely, the higher the temperature of a gas substance, the thicker the gas substance is.

## Capillarity

Cohesion force is an attraction force between molecules in a similar substance, while an attraction force between molecules of an unequal substance is called the Adhesion Force. For example, we pour water into a glass. Cohesion occurs when water molecules attract each other, while adhesion occurs when water molecules and glass molecules pull each other.

## Surface tension

Have you ever played soap bubbles? Soap bubble round. Funny. Can be blown. After flying, soap bubbles burst. Wow, it’s fun, a childhood game. Why is soap bubble round?

Please wake up in the morning, then notice the leaves around the house. Observe the droplets of dew attached to the leaves. It’s strange; dew drops are round. How can it be like that?

Surface tension occurs because the surface of the liquid tends to tighten so that the surface looks like a thin membrane. This is influenced by the cohesion force between water molecules. To better understand this explanation, consider the following illustration. We review the liquid in a container.

## Archimedes principle

A ship with a huge mass does not sink, while a stone that has a small size can sink. Why is that? The answer is straightforward if you understand the concept of buoyancy and Archimedes’ principle.

In everyday life, we will find that objects that are inserted into a fluid, like a rock, have a smaller weight than when objects are not in the liquid. You may find it difficult to lift a stone from the ground, but the same stone is effortlessly raised from the bottom of the seawater. This is due to the buoyant force. Buoyancy occurs due to differences in fluid pressure at different depths. The fluid pressure increases with depth, the thicker the fluid, the greater the pressure of the fluid. When an object is inserted into the fluid, there will be a difference in pressure between the fluid at the top of the object and fluid at the bottom of the object. Fluid located at the bottom of the object has a pressure higher than the fluid at the top of the object.