Thấu kính hội tụ là gì? Đặc điểm, cách vẽ và công thức tính chi tiết (Vật lý 9)

Converging lenses are part of the knowledge about lenses that students will learn in the Physics 9 program. So to better understand this knowledge from concepts, characteristics, drawing methods, calculation formulas, etc. Let's explore more clearly with timhieulichsuquancaugiay.edu.vn in the following article.

What is a converging lens?

In the definitions of optics, a lens is an optical device used to converge or diverge different beams of light. In a broader context, optical lenses are lenses that work with light and using traditional techniques.

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So what image does a converging lens produce? Unlike a diverging lens, a converging lens is a transparent lens, with the edges thinner than the middle. They are usually bounded by two spheres or a plane and a sphere.

This is a lens where the light beam after passing through the glass will converge at one point.

Characteristics of converging lenses

Characteristics of converging lenses include:

Light rays through the lens

• The light ray that reaches the lens is called the incident ray, and the emerging ray is the ray that refracts from the lens.

• When the incident ray reaches the optical center, the emerging ray continues to propagate straight in the direction of the incident ray.

• If an incident ray is parallel to the principal axis, the ray emerges through the focal point.

• The incident ray passes through the focal point and emerges parallel to the principal axis.

Shape of converging lens

A converging lens is limited by two spherical surfaces (one of which may be flat). The outer edges will be thinner than the middle.

Material used to make converging lenses

There are many materials that can be used to make a converging lens. Usually it is made from transparent materials, typically plastic or glass.

1. Plastic lens

This is the best material to create a converging lens because it is easy to use for all objects. Lenses made of plastic are also known as organic glass. This type is often used in making glasses for children or when playing sports because of its comfort, lightness, and extremely high break resistance.

2. Glass lens

The lenses are made of good quality natural mineral glass, which is extremely professionally graded and is the standard for these products. With this type of lens, consumers will spend less money than plastic glasses, especially they are extremely scratch resistant and suitable for cases of severe dystrophy.

How to identify converging lenses

To be able to recognize converging lenses, you can refer to the 3 ways below.

• Use your hands to identify them by the thickness of the center and the thickness of the edges. If the edge of the lens is thinner than the center, it is a converging lens.

• Point the near lens at the text on the book. If a lens can make the text seen through the lens larger than when seen directly on the book, it is a converging lens.

• Use a lens to catch light. Use a lens to catch the light of a lamp placed far away on the screen. If that beam of light converges on the screen, it is a converging lens.

Principal axis, optical center, focus, focal length of a converging lens

Below are concepts surrounding converging lenses to keep in mind

Principal axis of a converging lens

The main axis of a converging lens is the emerging ray, which can pass straight through the object and does not change direction when passing through the lens.

Optical center of a converging lens

For a converging lens, the optical center is the point where all light rays reaching this point can travel straight and not change direction. The optical center is denoted O.

Focal point of a converging lens

The focal point of a converging lens is denoted as F. They have an incident beam converging at one point and have an incident beam parallel to the main axis of the lens.

The transmission system of three special light rays is as follows:

• When the incident ray passes through optical center O, the emerging ray will propagate straight.

• The incident ray will be parallel to the main axis, then let the emerging ray pass through the focus F'

• The incident ray passing through focus F has the characteristic of being parallel to the principal axis.

Focal length of a converging lens

With the focal length of a converging lens, this is the distance from the focal point F of the lens, to the optical center O of the lens, denoted f and measured in cm.

How to draw a converging lens

To be able to draw a converging lens, let's learn the drawing steps below:

• Draw the main axis horizontally and denote it as (△).

• Construct the lens perpendicular to the principal axis. The point passing through the optical center is denoted by (O).

• There is an incident beam parallel to the principal axis of the converging lens. The main focus is the beam converging at a point on the main axis.

• The main foci are two points located on the main axis at a distance from the lens and symmetrical with the focus F on this side, we have the focus F' on the other side of the lens.

You can refer to the drawing below:

Image of an object through a converging lens

To construct image A'B' of segment AB through the lens (AB is perpendicular to the lens and point A is on the main axis △), just construct image B' of B with two of the three special light rays, from there B' is perpendicular to the main axis △, then we will get image A' of A.

Converging lens formula

Formula that relates the position of the object, the position of the image and the focal length of the lens

Sign conventions:

• Converging lens: f > 0

• Diverging lens: f

• The image is real: d' > 0

• The photo is virtual :d'

• The object is real: d > 0

Magnification formula of lens:

Sign conventions:

Formula to calculate the condensation of a lens:

In there:

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  n: refractive index of lens material

• D: condensation of the lens

• f: focal length of lens (m)

• R1; R2: radius of curved surfaces (R = ∞ for flat surfaces) (m)

Applications of converging lenses in life

As for converging lenses, they are used for many different purposes in life. Some applications include:

• Change the beam from parallel to converging light beam.

• Used as objective lens and eyepiece in microscope or telescope.

• Used as glass in cameras.

• Used as a magnifying glass.

• Used as glasses to treat farsightedness and presbyopia.

• Sometimes converging lenses are also used in many emergency situations such as creating a fire.

See more: What is a diverging lens? What are the characteristics of the image of an object through a diverging lens? (Physics 9)

Some converging lens exercises for grade 9

Question 1: A converging lens has the characteristic of turning parallel incident rays into what beam?

A. Reflected beam.

B. Converging beam of light.

C. Divergent emerging beam.

D. Other parallel emerging beams.

Question 2: When does an incident ray pass through the optical center of a converging lens?

A. Passes through the focal point and passes through the optical center

B. Parallel to the principal axis and passing through the focal point

C. Propagate straight in the direction of the incident ray

D. There is an extended line passing through the focus

Question 3: What materials are often used to make a lens?

A. Clear glass

B. Plastic is opaque

C. Aluminum

D. Water

Question 4: Which of the following statements is true about converging lenses?

A. The principal axis of a lens is any straight line of infinite length.

B. The optical center of the lens is equidistant from the two focal points.

C. The focal length of a lens depends on the area of ​​the lens.

D. The distance between two focal points is called the focal length of the lens.

Question 5: A bright object AB has the shape of an arrow 6cm high placed perpendicular to the principal axis of a converging lens, 15cm away from the lens. The lens has a focal length of 10 cm.

a/ Construct an image of the object through the lens

b/ Determine the size and position of the image

Question 6: A bright object AB has the shape of an arrow 6cm high placed perpendicular to the principal axis of a converging lens, 10cm away from the lens. The lens has a focal length of 15 cm.

a/ Construct an image of the object through the lens

b/ Determine the size and position of the image

Question 7: A bright object AB shaped like an arrow placed perpendicular to the main axis of a converging lens produces a real image 12 cm high, 30 cm away from the lens. The lens has a focal length of 10 cm.

Determine the size and position of the object

Question 8: Object AB is placed in front of a converging lens to produce an image. Please prove the lens formulas and .

Question 9: Indicate that Δ is the main axis of a lens, S is the light point, S' is the image of S created by the lens.

a) Is S' a real image or a virtual image?

b) What is the given lens?

c) Draw the picture and show how to construct it.

Question 10: Place a light object AB 1 cm high in front of a converging lens and 15 cm away from the lens, the real image is 0.6 cm high.

a. Draw pictures

b. Calculate the focal length of the lens and the distance from the image to the lens

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