Podsumowanie wiadomości z optyki

Before you start, prepare short statements explaining the following concepts.

1. Light sources.

2. Umbra and penumbra.

3. Reflection of lightreflection of lightReflection of light.

4. Flat mirrormirrormirror.

5. Concave mirror.

6. Construction of images in a spherical mirror.

7. Refraction of lightrefraction of lightRefraction of light.

8. Lenses.

9. Construction of images using lenses.

10. Sight and eye defects.

11. DispersiondispersionDispersion of light.

12. Speed of light.

OpticsopticsOptics is a branch of physics that studies light and its interaction with matter. Geometrical optics - explains the optical phenomena assuming that light propagates in a straight line in an optically homogeneous mediumoptically homogeneous mediumoptically homogeneous medium. In optics, we often use the concept of an optical medium.

The optically homogeneous medium - a medium that has all chemical and physical properties constant in its entire volume, e.g. a glass lenslenslens. In such a medium the light propagates in a straight line with the same speed in all directions.

The optically inhomogeneous medium - a medium, whose properties in various points of its volume are different, e.g. air with changing temperature.

1. Light sources

The sourcesourcesource of light is a body that emits light radiation in the visible light range.

We divide the light sources into natural and artificial ones.

Light sources can be:

• Point source - here the light source has very small dimensions with respect to the distance from the illuminated object. In the case of a point source, we assume that light rays come from a single point.

• Extended source - this means that its size is relatively large with respect to the distance to the illuminated object, e.g. the sun illuminating the Earth.

2. Umbra and penumbra

In an optically homogeneous mediumoptically homogeneous mediumoptically homogeneous medium, light propagates in a straight line. The straight line along which the light travels is called a rayrayray of light.

Umbra (shadow) is an area where light rays, stopped by an opaque object, do not reach.

Penumbra is an area illuminated by part of the light emitted by the sourcesourcesource. Penumbra is created when the object is illuminated by an extended light source.

3. Reflection of light

The rayrayray of light falling on the boundary of two optical media is reflected. The angleangle angle of incidence of the light ray is equal to the angle of reflection. We measure these angles relative to a straight line perpendicular to the boundary of these media at the point of incidence. This line is called normal (perpendicular incidence).

Normal, incident ray and reflected ray lie in one plane.

4. Flat mirror

Flat mirrors are smooth reflecting surfaces. They are usually made of glass covered with a layer of aluminium or silver or other metal.

The beam of parallel light rays falling on the mirrormirrormirror is reflected, and the rays in the reflected beam still remain parallel to each other.

If the reflecting surface is rough, then the directions of the reflected rays become divergent.

An image created in a flat mirror is upright and virtual. The virtual image is created behind the mirror at the intersection of the reflected rays extended behind the mirror.

5. Concave mirror

Spherical concave mirrormirrormirror is a mirror whose reflecting surface is the inner part of the sphere. A concave mirror can also be a segment of the paraboloid.

The optical system of the concave mirror consists of:

• main optical axis - a straight line passing through the centre of the curvature of the mirror (C) and its vertex (V).

• radius of curvature (R) - the radius of the sphere whose inner part is the reflecting surface of the mirror.

• focal point of the mirror (F) - if light rays travel parallel to the optical axis of the mirror, after reflection they will intersect at the same point located on the optical axis called the focal point.

• focal length (f) - is the distance between the focal point and the vertex of the mirror.

Relation between the focal length and the radius of curvature:

6. Construction of images in a spherical mirror

To construct an image in a spherical mirrormirrormirror, at least two of the following light rays are needed.

• Light rayrayray parallel to the optical axis, after reflection off the mirror passes through the focal point.

• Light ray traveling through the centre of the curvature of the mirror, after reflection returns back on the same path.

• Light ray passing through the focal point of the mirror, which after reflection becomes parallel to the optical axis. The image formed in a concave mirror can be magnified, the same size or reduced, real or virtual, upright or inverted.

7. Refraction of light

If the light ray reaches the boundary of two optically different media, its direction will change – this is refraction of lightrefraction of lightrefraction of light.

The angleangle angle between the direction of the incident rayrayray and the perpendicular to the boundary surface (normal) at the point of incidence is called the angle of incidence.

The angle of refraction is the angle between the perpendicular to the boundary surface (normal) at the point of refraction and the direction of the refracted ray.

The incident ray, perpendicular (normal) and the refracted ray lie in one plane.

The cause of the refraction phenomenon is the change in the speed of light propagation at the transition from one medium to another.

If the speed of light propagation in the first medium is greater than in the one to which the light passes, then the angle of incidence is greater than the angle of refraction.

If the speed of light propagation in the first medium is smaller than in the second medium, then the angleangle angle of incidence is smaller than the angle of refraction.

If the light falls perpendicular to the boundary of two media, then the direction of its path will not change (although the speed of its propagation in these media is different).

8. Lenses

The lenslenslens is a transparent body bounded by spherical, parabolic or cylindrical surfaces.
Lenses can both converge and diverge light. Lenses are divided into converging and diverging.
Examples of lens applications: glasses, magnifier, optical microscope, telescope.

The optical system of the lens consists of:

• Optical axis - a straight line passing through the centres of curvature of the surfaces from which the lens was created.

• Focal point (F) - if light rays travel parallel to the optical axis of the lens, after passing through the lens they will intersect at the same point located on the optical axis called the focal point.

• Focal length (f) - distance between the focal point and the centre of the lens.

9. Construction of images using lenses

At least two of the following light rays are needed for the construction of an image produced using a converging lens:

• Light rayrayray parallel to the optical axis - after passing through the lens it travels through the focal point.

• Light ray passing through the focal point - after passing through the lens, it travels parallel to the optical axis.

• Light ray passing through the centre of the lens - after passing through the lens it does not change direction (does not refract).

In the image construction in a diverging lens, two rays are used:

• Light ray parallel to the optical axis - after passing through the lens its extension passes through the virtual focal point.

• Light ray passing through the centre of the lens - after passing through the lens its direction (path) does not change.

The image formed in the converging lens can be magnified, the same size or reduced, real or virtual, upright or inverted. In diverging lenses, the resulting image is always upright, reduced and virtual.

10. Sight and eye defects

The basic sight organ is the eye in which the lenslenslens plays an important role. The rays of light after passing through the cornea fall on the converging lens, which creates a reduced, inverted and real image on the retina.

The human eye has the ability to accommodate, or the ability to match the eye to the distance in which the object being viewed is located. This is possible due to the lens's ability to change shape.

The minimum distance of good sight for a human eye with no eye defect is about 25 cm.

The most common eye defects are:

• Short‑sightedness (myopia) is a defect associated with the refraction of lightrefraction of lightrefraction of light through the lens (too convex) or too long distance from the retina to the lens (elongated eyeball). The image of a distant object is formed in front of the retina and is interpreted by the brain as indistinct and blurred. Diverging lenses are used to correct this defect.

• Long‑sightedness (hyperopia) is a defect associated with the refraction of light through the lens (too flattened) or too short distance from the retina to the lens (shortened eyeball). The image of a distant object is formed behind the retina and is interpreted by the brain as out of focus. Converging lenses are used to correct this defect.

11. Dispersion of light

White light is a mixture of colours from violet to red. We can see these colours by passing a beam of white light through the prism.

The prism is a transparent solid (usually made of glass), which is a polyhedron with a triangle base.

Light passing through the prism is refracted twice, for the first time on the boundary between air and glass (at the entrance to the prism), for the second time on the glass‑air boundary (at the exit from the prism).

During the passage through the prism the highest deviation from the primary direction has violet light, and the smallest - red.

The phenomenon of light separation into colours is called dispersiondispersiondispersion.

12. Speed of light

Light in a vacuum travels at approximately 300000 $\frac{{\displaystyle km}}{s}$.

In material media, the speed of light is smaller and different for various media, e.g. in water it is 225000 $\frac{{\displaystyle km}}{s}$, and in ice 229000 $\frac{{\displaystyle km}}{s}$.

Remember

• Geometrical opticsopticsoptics a branch of physics that explains the optical phenomena assuming that light propagates in a straight line in an optically homogeneous mediumoptically homogeneous mediumoptically homogeneous medium.

Exercises

Glossary

Keywords

optically homogeneous mediumoptically homogeneous mediumoptically homogeneous medium

opticsopticsoptics

reflection of lightreflection of lightreflection of light

refraction of lightrefraction of lightrefraction of light

sourcesourcesource