Basic Physics

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.

The difference between a plane mirror and a book when reflecting light is a plane mirror reflecting light towards our eyes at a certain angle, while the book reflects light toward our eyes at various angles. Why is that? To understand this, first learn two types of light reflection, namely specular reflection, and diffuse reflection.

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Specular reflection

Observe pictures A and B. Orange arrows represent light rays, and gray horizontal lines represent the plane mirror.

When light at A illuminates the surface of the plane mirror, the reflected light from the mirror cannot be seen at B and can only be seen in C. Reflections like this occur because the surface of the object is flat and very smooth so that all the light rays have the same angle of incidence and angle of reflection. Reflection like this is called the specular reflection.

Diffuse reflection

All rays come parallel, but if observed, the surface of the object is rough so that each Incident light rays and reflected light rays have a different angle of incidence and angle of reflection. When light illuminates the surface of an object at A, the light reflected by an object can be seen at B, and C. Reflection like this is called diffuse reflection.

Light is reflected in all directions not because the law of reflection of light does not apply but because the surface of the object is rough. Diffuse reflection occurs in objects that we can observe in everyday life such as stones, houses, books, cell phones, computers, vehicles, and others. Although the surface of objects such as books looks flat but actually on a microscopic scale the surface of the book is uneven.