Important Optics Concepts

The topics highlighted in this text refer to important concepts of Optics, notions that can generate doubts in Physics students.

Optics studies phenomena related to light, such as rainbows
Optics is the branch of Physics that is dedicated to the study of phenomena related to light. In this important part of Physics, the phenomena of refraction , reflection , diffraction , dispersion , formation of images in mirrors and lenses , interaction between light and objects and the various optical instruments are studied .

The correct understanding of Optics concepts is fundamental for a perfect understanding of the phenomena and also for the resolution of the calculations on this part of Physics. The highlighted points below explain some concepts that can often cause doubts among students.

1. Color of objects

Color is not a characteristic of objects, but is defined by the light that illuminates them. Depending on the type of light that illuminates an object, monochromatic (one color) or polychromatic (white light), it can appear in different colors.

Colored objects have the ability to reflect only one color of light . A body that is red can only reflect red monochromatic light, and any other incident monochromatic light will be absorbed. Therefore, if a red object is illuminated by a source that emits yellow monochromatic light, it will absorb the incident radiation and reflect nothing, appearing as a blackbody.

In the following image, the plant is being illuminated by white (polychromatic) light coming only from the Sun. Note that the flower and leaf absorb radiation and reflect only the color of light that defines them. If the plant were fully illuminated by a yellow monochromatic source, the flower and leaf would be perceived as black, as they absorb all the incident radiation.

An object that appears as black absorbs any type of incident light . White objects have the ability to reflect any type of incident radiation, so they can appear in any color.

2. Real image versus virtual image

The difference between real and virtual images is crucial for understanding the different types of images formed by spherical mirrors and lenses.

Virtual image : The formation of the virtual image occurs when the ray of light coming from a certain body hits a reflecting or refracting object and is diverged, scattered. The virtual image always has the same orientation as the object and cannot be projected, it exists only in the mirror.

Real image : the formation of the real image occurs when the rays of light coming from a certain body hit a reflective or refracting object and are converged and forwarded to a common point, where there will be the formation of an image. The real image always has the opposite orientation to the object and is projectable, being able to be launched on walls and screens, for example.

3. Does it approach or depart from normal?

When light changes from a medium from a higher to one with a lower refractive index , its speed is increased when it comes into contact with the less refractive medium. Thus, the light ray moves away from the normal line (N), which shows the increase in speed.

The light approaches the normal line, which shows the increase in the refractive index

On the other hand, when light changes from a lower medium to a higher refractive index medium, its speed is slowed when it comes into contact with the more refractive medium. Thus, the ray of light approaches the normal line (N), which shows the decrease in speed.

To understand more about the effects of refraction on light, click here!

4. Angles of incidence, reflection and refraction

When incident on the surface of separation between two different media or on a reflective surface, the light forms an angle of incidence. The angle of incidence is only considered as the angle formed between the incident light ray and the normal line. The same rule applies to angles of reflection and refraction. To learn more about this subject, click here !

The terms i, r and r’ represent the angles of incidence, reflection and refraction, respectively.

5. The lens degree

The lenses that form the glasses, depending on each vision problem , can be converging or diverging . Convergents have the ability to focus light to a single point. Divergent ones scatter incident light. The light bending capacity of a lens is measured by a quantity called vergence (V), determined by the inverse of the focus of a lens.

The unit of measurement for vergence is the diopter (di), which is the reciprocal of a meter (1 di = 1 m – 1 ). Everyday, the term degree is used to refer to the vergence of a lens.

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