Before the invention of the clock, people determined time by the length of the shadow. What are shadows? How are shadows formed?

A shadow represents an absence of light. When an object blocks the passage of light, the other side of that object will get less light, forming a dark shape of the object behind it. This darkness is called a shadow.

Two factors are required for shadow formation: light and an opaque object.

The rectilinear property of light states that light travels in a straight line. This can be understood from the pinhole experiment.

Holes are made through the middle of three cardboard sheets, a, b, and c. The sheets are then placed so that the holes are directly in line with a source of light. When a, b, and c are aligned you can see the light through the holes. In the bottom image, the second cardboard (b) is placed higher than the first and the third, such that the cardboard blocks the light. Now you cannot see the light if you look through the holes. This proves that light can travel only in a straight line.

Types of objects

The type of object determines its ability to form shadows when placed in the path of light.

1. When light reaches a transparent object, it can travel unhindered.

2. When light reaches a translucent object, it can pass through partially.

3. When light reaches an opaque object, it cannot pass through, and shadows are formed.

Remember, when light reaches a shiny object, it reflects. For example: a mirror.

Factors that determine the size, shape, and depth of shadow

1. Source of light: Shadows are sharp when the light source is bright, like direct sunlight. Shadows can also be formed under artificial light. Shadows are sharper in the bright beam of a flashlight than in overhead light. If the light source is not bright, the shadows will be blurred.

2. Position of light: When the light falls at 90 degrees perpendicular to the object, the shadows are shorter. When the angle between a light beam and the object increases, then the shadow gets longer.

3. Size and shape of the object: The shape and the size of the object determine the boundaries of the shadow. For example, if you hold a ball in the path of light, then its shadow will be circular, and if you hold a rectangular box, then its shadow will have that shape.

The position of light determines the shape of the shadow

Step out on a sunny day and observe your shadow at several times throughout the entire day. You will be fascinated by how the shadow changes over time. The length of the shadow is determined by the position of the sun in the sky. When the sun is low on the horizon in the morning and evening, long shadows are formed. During mid-day, when the sun is high in the sky, the shadows are much shorter.

An eclipse is caused when celestial bodies form shadows. There are two types of eclipses: the solar and the lunar eclipse. Eclipses occur when the Sun, Earth, and Moon are aligned in the same straight line. When the Moon is between the Sun and the Earth, it throws its shadow on the Earth. This is called a solar eclipse. When the Earth comes between the Sun and the Moon, it throws its shadow on the Moon. This is called a lunar eclipse.

A shadow has three parts: the umbra, penumbra, and antumbra.

Umbra: Formed by point light sources, like a flashlight, it is the innermost region of the shadow. In this darkest part of the shadow there is no light. An observer within an umbra will perceive a total eclipse.

Penumbra: The region of shadow in which only a proportion of light is blocked by the object. If the observer is located within a penumbra, he sees only a partial eclipse.

Antumbra: The region of the shadow in which the object seems to be completely within the path of light. If the observer is placed in the antumbra, he will see an annular eclipse, which appears as a thin outer ring of the Sun’s disk that is not covered by the Moon.

For a non-point source of light, such as a lamp, the shadow is divided into umbra, penumbra, and antumbra. In contrast, for a point source of light, such as a laser, the shadow has only umbra.

Yes, they can.

Are you surprised?

To understand colored shadows, we need to understand light. We know that white light is a combination of all colors of light.

Imagine that red, blue, and yellow lights are turned on in a dark room. An object is placed in front of the three lights. What will you see? Not a dark shadow, but a shadow of green, magenta, and cyan. The colors in the shadow are the secondary colors formed by the overlapping of primary colors. So, our shadow is black only if we use white light.

Shadows can be formed in darkness, but they appear diffused. A shadow is a spot where the amount of light is reduced compared to the surrounding area. In bright light, the contrast between the shadow and the surrounding area is pronounced, so the shadow appears sharp. In dim light, the contrast is less, so the shadow appears diffused.

In a non-point light, such as overhead light, the shadow formed has three regions: the umbra, penumbra, and antumbra. When the penumbra of two shadows overlap, the shadows appear to attract each other and merge. This is called the shadow blister effect.

Can an object have more than one shadow?

Yes, an object can form more than one shadow when there is more than one light source. In a floodlit sports field, two or more shadows may be formed around each object depending on the number of lights.

Conclusion

Shadows are formed when light cannot penetrate an object. There are three regions in a shadow: umbra, penumbra, and antumbra. Not all shadows are dark; some can be colored depending on the source of light.

Glossary

1. Light: an electromagnetic wave comprised of different wavelengths
2. Umbra: the innermost region of a shadow
3. Reflection: When a ray of light strikes a smooth, polished surface, the light ray bounces back.
4. Transparent object: allows light to pass through easily
5. Translucent object: partially allows light to pass through it
6. Opaque object: does not allows any light to pass through it