What Is Light?



Light

Note: This post addresses the question at “GCE O Level” level. For this topic of “Reflection & Refraction Of Light”, we will only be concerned about light as an electromagnetic wave.

In terms of classical wave theory, light is an electromagnetic wave that is capable of passing through free space or through a material medium in the form of varying electric and magnetic fields. In the absence of influence, the wave travels straight.

The visibility of an object is contingent upon the entry of light into your eyes. Certain entities, like the Sun, electric lamps, and candles, possess the ability to emit their own light, categorizing them as luminous sources. However, a majority of the objects that you observe do not inherently produce light; instead, they reflect light originating from a luminous source. These objects are termed non-luminous, and examples include this article, yourself, and the Moon.

Laser As A Light Source

A distinctive type of light source is the laser, introduced in 1960. Unlike conventional light sources, in laser light sources, the excited atoms collaborate to produce a focused and intensely bright light beam. The laser finds a myriad of applications across various fields. It is employed in industrial settings for precision cutting of plate metal, in scanners for reading barcodes during retail and library transactions, in CD players, within optical fiber telecommunication systems, in intricate medical procedures involving the eye or inner ear, as well as in printing, surveying, and range-finding operations.

Luminous sources emit light when their atoms are stimulated or ‘excited’ due to the absorption of energy. In instances like a light bulb, the energy is derived from electricity. The light emitted by these excited atoms is typically dispersed randomly in most luminous sources.

Beams Of Light

Sunlight filtering through branches and the light emitted by a cinema projector en route to the screen imply the straight-line trajectory of light. These beams become visible as dust particles in the air reflect light toward our eyes. The path along which light is traveling is termed a “ray” and is symbolized in diagrams by a straight line adorned with an arrow. A “beam” refers to a flow of light and is depicted by multiple rays. A beam can exhibit parallel, diverging (dispersing outward), or converging (contracting) characteristics.

Speed Of Light

The evidence of light’s significantly faster speed compared to sound is evident in thunderstorms. The lightning flash becomes visible before the sound of thunder is heard. The duration of this time gap increases with the distance of the observer from the storm. Although light doesn’t travel instantaneously between points, it does so in an extremely brief period. Its velocity is approximately 1 million times faster than that of sound.

Reflection & Refraction Of Light

More Details In Reflection Of Light & Refraction Of Light

When light is incident in a perpendicular direction on the surface, such as a piece of glass, part is transmitted and part is reflected. This happens at any surface that forms the boundary between two transparent mediums of different refractive indices.

Pinhole Camera

1920px Pinhole camera.svg

A pinhole camera is one of the simplest forms of cameras. It does not use a lens; instead, it forms an image through a small hole, the pinhole. The basic principle behind the pinhole camera is that light travels in straight lines. When light rays from an object pass through a tiny hole in a dark box or container, they do not scatter but cross and reform on the opposite side of the hole, projecting an inverted and reversed image of the object outside onto the surface facing the hole.

Creation Of The Pinhole Camera

To make a pinhole camera, you need a light-tight box or container, a piece of aluminum foil to make the pinhole, and some photosensitive paper or film to capture the image. Here’s a simplified process:

  1. Make a small hole in the middle of one side of the box using a pin. This is your pinhole.
  2. Cover the pinhole on the inside with a small piece of aluminum foil and secure it with tape. Then, poke a very small hole in the foil with the pin.
  3. On the inside of the box, opposite the pinhole, place the photosensitive paper or film where the image will be projected.
  4. Make sure the box is completely light-tight except for the pinhole.
  5. To take a picture, aim the pinhole towards the scene or object you want to capture, and cover and uncover the hole to control the exposure time.

How Does The Image From A Pinhole Camera Differ From The Object?

  1. Inverted: The image formed is upside down compared to the actual object. This occurs because the light rays from the top of the object and the bottom cross at the pinhole.
  2. Reversed: The image is also laterally inverted (left-right reversed) due to the way light travels through the pinhole.
  3. Dimmer: The image is usually dimmer than the object. This is because the pinhole allows only a small amount of light to enter, reducing the brightness of the image.

What Is The Effect Of Moving The Camera Closer To The Object?

Moving the pinhole camera closer to the object makes the image larger but less sharp. This is because a larger portion of the object’s details is captured, but since the pinhole spreads the light over a larger area, the image’s clarity decreases.

What Is The Effects Of Making The Pinhole In A Pinhole Camera Larger?

  1. Brightness: The image becomes brighter. A larger pinhole allows more light to enter the camera, increasing the brightness of the image.
  2. Sharpness: The image becomes less sharp. A larger pinhole causes more light rays from different parts of the object to overlap on the image plane, blurring the details.
  3. Size of the Image: The size of the image is not directly affected by the size of the pinhole. It primarily depends on the distance between the pinhole and the photosensitive surface and the distance from the object.

However, a larger pinhole can lead to a less distinct image, making it seem less well-defined.

Understanding these principles of the pinhole camera can offer insights into the basics of photography and the fundamental properties of light.

Further Readings

In terms of the modern quantum theory, electromagnetic radiation, such as light, is a flow of photons (“small packets of light”) through space. This means that light is made up of particles.

Particles and waves are two entirely different things. How can light be both a particle and a wave? If you are interested, check out my post on Wave particle duality and/or google for the term.

When light is incident in a perpendicular direction on the surface, such as a piece of glass, part is transmitted and part is reflected. This happens at any surface that forms the boundary between two transparent mediums of different refractive indices.


Worked Examples

Example 1

What is the nature of light according to classical wave theory?

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According to classical wave theory, light is an electromagnetic wave that can travel through free space or a material medium in the form of varying electric and magnetic fields. It travels in a straight line in the absence of any influence.

Example 2

How do luminous and non-luminous objects differ in terms of light?

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Luminous objects are sources that emit their own light due to the stimulation or ‘excitement’ of their atoms, examples include the Sun, electric lamps, and candles. Non-luminous objects do not produce their own light; instead, they reflect light from luminous sources. Examples of non-luminous objects are articles, humans, and the Moon.

Example 3

Describe the unique characteristics of laser light compared to conventional light sources.

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Laser light is produced when excited atoms work together to emit a focused and intensely bright beam of light, which is unlike the randomly dispersed light emitted by most conventional light sources. Lasers find applications in various fields such as precision cutting, barcode scanning, telecommunications, medical procedures, and more due to their focused intensity.

Example 4

Explain the principle behind the pinhole camera and how it forms an image.

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The pinhole camera operates on the principle that light travels in straight lines. When light rays from an object pass through a tiny hole (the pinhole) in a dark box, they do not scatter but cross and reform on the opposite side, projecting an inverted and reversed image of the object outside onto the surface facing the hole.

Example 5

What are the effects of altering the size of the pinhole in a pinhole camera?

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Changing the size of the pinhole affects the brightness and sharpness of the image. A larger pinhole increases the brightness of the image because it allows more light to enter but reduces the sharpness as more light rays from different parts of the object overlap on the image plane, blurring the details. The size of the image primarily depends on the distance between the pinhole and the photosensitive surface and the distance from the object, but a larger pinhole can make the image seem less well-defined due to decreased distinctness.

Example 6

What distinguishes luminous sources from non-luminous sources? Provide examples of each.

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Luminous sources emit their own light due to excited atoms, such as the Sun and electric lamps. Non-luminous sources, on the other hand, reflect light from luminous sources, like the Moon and objects such as this article.

Example 7

What factors influence the brightness and sharpness of an image captured by a pinhole camera?

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The size of the pinhole affects the brightness and sharpness of the image. A larger pinhole allows more light to enter, making the image brighter but less sharp due to overlapping light rays. Additionally, the distance between the pinhole and the photosensitive surface impacts the image’s size and clarity.

Example 8

How does the movement of a pinhole camera towards an object affect the resulting image?

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Moving the pinhole camera closer to an object makes the image larger but less sharp. This occurs because the closer proximity captures more details of the object, but the spreading of light over a larger area reduces the clarity of the image.

Example 9

Compare the speed of light with the speed of sound. How does this speed difference manifest in everyday situations?

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Light travels approximately 1 million times faster than sound. This speed difference is evident in phenomena like thunderstorms, where lightning flashes are seen before the accompanying sound of thunder is heard. The duration of this gap increases with the observer’s distance from the storm, highlighting the remarkable swiftness of light compared to sound.


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