- does this mean that
2. Tài liệu tập huấn
36 Tài liệu tập huấn giảng viên dạy chuyên ngành bằng tiếng Anh
Bài thực hành mẫu “Các định luật thấu kính và quang cụ” (Laws of lenses and optical instruments).
LAWS OF LENSES AND OPTICAL INSTRUMENTS
I. RELATED TOPICS
+ Law of lenses: the thin lenses formula;
+ Path of a ray through a system;
+ Focal length, object distance, real image and virtual image.
+ Measuring the focal length of convex and concave lens;
+ Simple models of optical instruments: slides projector, microscope and telescope.
II. THEORY
The relationship between the focal length of a lens, the object distance and the image distance is obtained from geometrical optics. Three particular rays, the focal ray, the parallel ray and the central ray, are used to construct the image (Fig. 1).
Figure 1. Image construction with three principal rays.
From the laws of similar triangles, we have
37 Tài liệu tập huấn giảng viên dạy chuyên ngành bằng tiếng Anh
(2)
where is the image size and is the object size.
By transforming we obtain the lens formula
(3)
(4)
1. From the values of and measured in Task 1 we calculate , the average value of and its average deviation.
2. Since (the object distance in case I = image distance in case II) and since , then
(5)
(6)
(see Fig. 3).
If we solve the equations for and we obtain
(7)
(8)
Substituting (7) and (8) into the lens formula (4) gives
38 Tài liệu tập huấn giảng viên dạy chuyên ngành bằng tiếng Anh
The focal length of the convex lens can therefore be determined from the measured values of and .
If we now use a lens system of focal length . Consisting of the
convex lens already measured (focal length ) and a concave lens, and
carry out the measurement in the same way, we obtain the following for
the focal length of the concave lens :
(10)
(11)
Here we assume that
(12)
as otherwise no real images would be produced.
3.1 The magnification is obtained from the relationship between object size and image size as follow
(13)
3.2 The overall magnification is obtained by multiplying the magnification due to the objective (Fig. 5)
(14)
39 Tài liệu tập huấn giảng viên dạy chuyên ngành bằng tiếng Anh
3.3 The objective L1 provides a real, inverted image of size of a very distant object, and this image is observed through the eyepiece L2. The angular magnification (for small angles) is
(15)
3.4 A concave lens is placed in the path of the ray in front of the real first image produced by objective L1 so that the focal points F1 and F2 coincide. The eye then sees a virtual, upright image. The magnification is once again
(15)
Note
The markings on the lenses used to measure focal length should be removed and replaced by a code, e.g. coloured dots or etched lines, known only to the instructor.
