Basic Photography Course

the index of refraction for that substance; for example,
light travels about 1 1/2 times as fast in air as it does in
glass, so the index of refraction for glass is about 1.5.

Refraction, or change of direction, always follows

a simple rule.

"In passing from one transparent substance into

another of greater density, refraction is toward the
normal. In passing from one transparent substance into
another of lesser density, refraction is away from the
normal."

In this rule the normal is defined as a line

perpendicular (90°) to the surface between the mediums.

Refraction is shown in figure 1-8. The ray of light

(AB) strikes the glass at an oblique angle. Since the glass
is denser than air, the ray of light is bent toward the
normal (RS) and emerges from the glass at (C). Upon
entering the air again, the ray is bent away from normal
(RS) and travels along the path (CD).

All rays striking the glass at an angle other than

perpendicular are refracted. In the case of the
perpendicular ray (ME) that enters the glass normal to
the surface, no refraction takes place and the ray
continues through the glass and into the air in a straight
line.

DISPERSION

The speed of light in a medium depends on the

wavelength of the light. As light enters a more dense
medium, the short waves, such as blue, are slowed more
than the long waves, such as red. Thus the index of
refraction of a medium varies with the wavelength, and
the different colors of light are bent different amounts.
This changing index of refraction or the breaking up of
white light into its component colors is called

dispersion. This then ties in with the previous discussion
of the colors of light where we saw the way a prism
creates a spectrum from white light. The prism is able
to create this spectrum because of dispersion.

DIFFRACTION

We have said that light travels in a straight line. Well,

that is not always true. An exception to this rule occurs
when light travels close to an opaque edge. Because of
the wave nature of their travel, light rays passing near
an opaque edge are bent ever so slightly (fig. 1-9). This
bending is called diffraction and is evidenced by the
formation of a shadow with a fuzzy edge when light

passes an opaque object. In this case, the outside edge
of the shadow is light and indistinct, but it gradually
darkens into the true black of the shadow that indicates
that some of the light is scattered into the shadow area.

Unlike refraction, in diffraction the long wave-

lengths of light are bent the most.

Diffraction is important to the photographer when

the light passes the edges of a lens diaphragm. When the
lens diaphragm is opened fully, the amount (actually the

percentage) of diffracted light is quite small. But when
the diaphragm is closed to a small opening, the
percentage of diffracted light is quite large and reduces
the sharpness of the image formed by the lens. In other
words, a small aperture opening interferes with the
image-forming light more than a large aperture does.

POLARIZATION

Energy in the form of wave motion radiates from its

source and travels through a medium. For example,

1-6

contents

Basic Photography Course

<- previous page

next page ->


DOFMaster for Windows On-line Depth of Field Calculator DOFMaster for Mobile Devices On-line Depth of Field Table Hyperfocal Distance Chart Articles FAQ Recommended Books Support Contact Links Home As an Amazon Associate I earn from qualifying purchases.	contents <- previous page next page -> Figure 1-8.The law of refraction. Figure 1-9.Diffraction. the index of refraction for that substance; for example, light travels about 1 1/2 times as fast in air as it does in glass, so the index of refraction for glass is about 1.5. Refraction, or change of direction, always follows a simple rule. "In passing from one transparent substance into another of greater density, refraction is toward the normal. In passing from one transparent substance into another of lesser density, refraction is away from the normal." In this rule the normal is defined as a line perpendicular (90°) to the surface between the mediums. Refraction is shown in figure 1-8. The ray of light (AB) strikes the glass at an oblique angle. Since the glass is denser than air, the ray of light is bent toward the normal (RS) and emerges from the glass at (C). Upon entering the air again, the ray is bent away from normal (RS) and travels along the path (CD). All rays striking the glass at an angle other than perpendicular are refracted. In the case of the perpendicular ray (ME) that enters the glass normal to the surface, no refraction takes place and the ray continues through the glass and into the air in a straight line. DISPERSION The speed of light in a medium depends on the wavelength of the light. As light enters a more dense medium, the short waves, such as blue, are slowed more than the long waves, such as red. Thus the index of refraction of a medium varies with the wavelength, and the different colors of light are bent different amounts. This changing index of refraction or the breaking up of white light into its component colors is called dispersion. This then ties in with the previous discussion of the colors of light where we saw the way a prism creates a spectrum from white light. The prism is able to create this spectrum because of dispersion. DIFFRACTION We have said that light travels in a straight line. Well, that is not always true. An exception to this rule occurs when light travels close to an opaque edge. Because of the wave nature of their travel, light rays passing near an opaque edge are bent ever so slightly (fig. 1-9). This bending is called diffraction and is evidenced by the formation of a shadow with a fuzzy edge when light passes an opaque object. In this case, the outside edge of the shadow is light and indistinct, but it gradually darkens into the true black of the shadow that indicates that some of the light is scattered into the shadow area. Unlike refraction, in diffraction the long wave- lengths of light are bent the most. Diffraction is important to the photographer when the light passes the edges of a lens diaphragm. When the lens diaphragm is opened fully, the amount (actually the percentage) of diffracted light is quite small. But when the diaphragm is closed to a small opening, the percentage of diffracted light is quite large and reduces the sharpness of the image formed by the lens. In other words, a small aperture opening interferes with the image-forming light more than a large aperture does. POLARIZATION Energy in the form of wave motion radiates from its source and travels through a medium. For example, 1-6 contents Basic Photography Course <- previous page next page ->	As an Amazon Associate I earn from qualifying purchases.
	WWW.DOFMASTER.COM © 2006 Don Fleming. All rights reserved.