The focal length of a single lens is the distance from the main point to the focus. The design wavelength of the lens is 546.1 nm (E-line of green mercury line). Since the focal length changes with the wavelength, when it is used for other wavelengths, the focal length also changes.
The lens with m at the end of the model is plated with anti reflection multilayer film.
Difference with biconvex lens
A biconvex lens. Light incident on both sides has the same refractive index. The focal length is the same.
The light incident on both sides of a plane convex lens has a slightly different refractive index. So in fact, the focal lengths on both sides are different. When in use, it cannot be reversed.
Effect of plano convex lens
This kind of lens is used to focus parallel light or convert point light source into parallel light.
Reasonable selection of materials (texture, bubbles, impurities, uniformity); When grinding, it is considered that there is no scattering (scratch, dent, gloss) when it is used for interference light.
Whether the specifications are coated or not and anti reflection multilayer lens coated with visible band domain.
Telescope:
In May 1609, the famous scientist Galileo visited Venice at the age of 45°，There he heard the story of the Dutch man-made telescope and was very excited about it.
Galileo immediately began to build his own telescope. He installed a flat convex lens and a flat concave lens at both ends of a lead pipe. The end of a flat concave lens close to the eye is called an eyepiece. A flat convex lens is called an objective lens when it is close to one end of the observation object. When he observed objects with his homemade telescope, objects in the distance were magnified many times.
Although Galileo was not the first person to invent the telescope, he became the first person to aim the telescope at the stars with the sensitivity of his scientists. Since then, the astronomical telescope was born, astronomy has obtained a new weapon to explore the universe, and has made amazing discoveries one after another.
Calculation example
Calculation of focal length formula of plane convex lens with certain thickness
For example, the focal length of a flat convex lens with a thickness of 5 and a radius of 20
(1) In the case of a convex lens, the circle radius of the plane:
1、Calculate the spherical radius of convex lens: R ^ 2 = 20 ^ 2 + (R-5) ^ 2, r = 42.5;
2、 Make a straight line AA parallel to the optical axis, the convex lens spherical surface is at point a, the straight line AA “is perpendicular to the optical axis, and the intersection with the optical axis is a”, the length of the straight line section AA “is h, and make straight lines OA and FA, where o is the center of the ball and F is the focus, pass through point a as the tangent ab of the convex lens spherical surface, and the optical axis is at point B, then OA is perpendicular to AB, and the straight line OA is the normal of the incident light AA ‘;
3、Calculate the sinusoidal value of the incident angle: because AA ‘is parallel to the optical axis of, the incident angle a = ∠ AOB, sin (a) = sin (AOB) = H / R, ∠ AOB = arcsin (H / R);
4、If the refractive index of the material used for the convex lens is n, then the sinusoidal value of the exit angle sin (b) = sin (a) / N = (H / R) / N = (h * n) / R, that is, sin (oaf) = (h * n) / R, ∠ oaf = arcsin [(h * n) / R];
5、∠AFO=∠AOB-∠OAF=ArcSin[(h*n)/r]-ArcSin(h/r)，tg(AFO)=tg{ArcSin[(h*n)/r]-ArcSin(h/r)}=AA”/FA”=h/FA”，FA”=h/tg{ArcSin[(h*n)/r]-ArcSin(h/r)}；
6、One side is a thin convex lens with a plane, and the optical center can be approximately regarded as the center of the plane, so the focal length f = of + (R-5) = (FA “- OA”) + (R-5), and OA ^ 2 = OA “^ 2 + AA” ^ 2, that is, OA “= √ (OA ^ 2-AA” ^ 2) = √ (R ^ 2-h ^ 2);\
7、Finally, by substituting Fa “and OA”, the focal length f can be calculated by simplifying the calculation.
(2) If it is the spherical radius of the convex lens, the first step in the above steps can be omitted and calculated directly from the second step.
Radius of curvature
Newton ring instrument is composed of the flat convex lens to be measured (the curvature radius of the convex surface is about 200 ~ 700cm) and the polished flat glass plate superimposed in the metal frame. When the convex surface of a flat convex lens with large curvature radius contacts a polished flat glass plate, an air film will be formed between the convex surface of the lens and the flat glass plate, with the same thickness at the same distance from the contact point. If the wavelength is λ When the monochromatic parallel light is projected onto the device, the light waves reflected by the upper and lower surfaces of the air film will interfere with each other, and the interference fringes formed are the tracks of equal thickness points of the film. This interference is a kind of equal thickness interference.
When observing in the reflection direction, you will see a group of bright and dark ring-shaped interference fringes centered on the contact point, and the center is a dark spot. If you observe in the lens direction, the light intensity distribution of the interference ring and the interference ring of the reflected light is just complementary. The center is a bright spot. The original bright ring becomes a dark ring, and the dark ring becomes a bright ring. This interference phenomenon was first discovered by Newton, so it is called Newton’s ring.
Principle
Newton ring
The Newton ring instrument is composed of a flat convex lens to be measured (the radius of curvature of the convex surface is about 200 ~ 700cm) l and a polished flat glass plate P superimposed in the metal frame f, as shown in Fig. 1. There are three spirals h on the edge of the frame to adjust the contact between L and P to change the shape and position of the interference ring. When adjusting h, the screw shall not be rotated too tightly to avoid elastic deformation of the lens caused by excessive contact pressure and even damage to the lens.
When the convex surface of a flat convex lens with large curvature radius contacts a polished flat glass plate, an air film will be formed between the convex surface of the lens and the flat glass plate, with the same thickness at the same distance from the contact point. As shown in Figure 2. If the wavelength is λ When the monochromatic parallel light is projected onto the device, the light waves reflected by the upper and lower surfaces of the air film will interfere with each other, and the interference fringes formed are the tracks of equal thickness points of the film. This interference is a kind of equal thickness interference.
When observing in the reflection direction, you will see a group of bright and dark annular interference fringes centered on the contact point, and the center is a dark spot (Fig. 3 (a)). If you observe in the lens direction, the light intensity distribution of the interference ring and the interference ring of the reflected light is just complementary. The center is a bright spot, the original bright ring becomes a dark ring, and the dark ring becomes a bright ring (Fig. 3 (b)). This interference phenomenon was first discovered by Newton, So it is called Newton ring.
