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Diagram a (prism) shows a clear pyramid with light entering one surface. The light leaving the other surface is bent and is the refracted light. A dotted line indicates the path the original light beam would have taken had it not bent. The region above the dotted line is labeled high refractive index; the region below the line is labeled low refractive index. Diagram b (convex lens) shows a lens with a bulge in the center. Light enters one either side of the dome and is focused to a point past the lens and in line with the center of the dome. The point at which the light focuses is the focal point; the distance from the focal point to the center of the lens is the focal length. Diagram c (concave lens) shows a lens that curves inward on either side. Light entering this lens is bent outwards, away from the center of the lens’s curve. A dotted line shows the linear path backwards for each of the bent light beams. The point at which all the dotted lines meet (which is on the other side of the lens) is the focal point.
(a) A lens is like a collection of prisms, such as the one shown here. (b) When light passes through a convex lens, it is refracted toward a focal point on the other side of the lens. The focal length is the distance to the focal point. (c) Light passing through a concave lens is refracted away from a focal point in front of the lens.

The human eye contains a lens that enables us to see images. This lens focuses the light reflecting off of objects in front of the eye onto the surface of the retina, which is like a screen in the back of the eye. Artificial lenses placed in front of the eye (contact lenses, glasses, or microscopic lenses) focus light before it is focused (again) by the lens of the eye, manipulating the image that ends up on the retina (e.g., by making it appear larger).

Images are commonly manipulated by controlling the distances between the object, the lens, and the screen, as well as the curvature of the lens. For example, for a given amount of curvature, when an object is closer to the lens, the focal points are farther from the lens. As a result, it is often necessary to manipulate these distances to create a focused image on a screen. Similarly, more curvature creates image points closer to the lens and a larger image when the image is in focus. This property is often described in terms of the focal distance, or distance to the focal point.

  • Explain how a lens focuses light at the image point.
  • Name some factors that affect the focal length of a lens.

Electromagnetic spectrum and color

Visible light is just one form of electromagnetic radiation (EMR) , a type of energy that is all around us. Other forms of EMR include microwaves, X-rays, and radio waves, among others. The different types of EMR fall on the electromagnetic spectrum, which is defined in terms of wavelength and frequency. The spectrum of visible light occupies a relatively small range of frequencies between infrared and ultraviolet light ( [link] ).

A series of scales indicate that the image shows the lowest wavelength (10 superscript -18 m) on the left and the highest wavelength (10 superscript 6 m) on the right. The frequencies range from over 10 superscript 24 Hzon the left to 1 Hz on the right. The energies range from 10 superscript 12 ev on the left to 10 superscript -12 on the right. The types of radiation listed above these scales (from left to right) is: cosmic radiation, gamma rays, X-rays, ultra-violet, visible, infrared, Terahertz radiation, radar, television and radion broadcasting, and AC circuits. The visible light portion of the spectrum is pulled out and shows blue light at 400 nm, green light at 500 nm, yellow light at 600 nm, and red light at 700 nm.
The electromagnetic spectrum ranges from high-frequency gamma rays to low-frequency radio waves. Visible light is the relatively small range of electromagnetic frequencies that can be sensed by the human eye. On the electromagnetic spectrum, visible light falls between ultraviolet and infrared light. (credit: modification of work by Johannes Ahlmann)

Whereas wavelength represents the distance between adjacent peaks of a light wave, frequency, in a simplified definition, represents the rate of oscillation. Waves with higher frequencies have shorter wavelengths and, therefore, have more oscillations per unit time than lower-frequency waves. Higher-frequency waves also contain more energy than lower-frequency waves. This energy is delivered as elementary particles called photons. Higher-frequency waves deliver more energetic photons than lower-frequency waves.

Photons with different energies interact differently with the retina. In the spectrum of visible light, each color corresponds to a particular frequency and wavelength ( [link] ).The lowest frequency of visible light appears as the color red, whereas the highest appears as the color violet. When the retina receives visible light of many different frequencies, we perceive this as white light. However, white light can be separated into its component colors using refraction. If we pass white light through a prism, different colors will be refracted in different directions, creating a rainbow-like spectrum on a screen behind the prism. This separation of colors is called dispersion , and it occurs because, for a given material, the refractive index is different for different frequencies of light.

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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