# Ray Optics and Optical Instruments

Ray Optics and Optical Instruments
Lesson Plan for Class XII
Learning outcomes
? Students will understand the difference between virtual and real images.
? Students will extend their previous knowledge of reflection and refraction at plane surfaces to curved surfaces.
? They will further learn to apply these to understand how lenses form images.
? Students will learn about combination of lenses, and how they can be used to achieve desired results.
? Students will learn about the construction and working of some basic optical instruments: the compound microscope and the refracting telescope.
Key Ideas
? Types of images: real and virtual
? Image formation
? Magnification: lateral and angular
The Opening
Review the laws of reflection and refraction that the students are already familiar with. Discuss the two types of images, real and virtual, with attention to how they are formed. Also discuss ray tracing, another concept that they have learnt earlier.
Phase 1
Help them apply the laws of refraction and refraction to curved surfaces and derive the mathematical relations. Draw the figure on the board, and let them try to figure out the mathematical relations, with minimal guidance from the teacher. Pay special attention to rules of sign conventions, as they have not studied this before, and often tend to get confused.
Using these laws, ask them to derive mathematical expressions for lenses (lens formula and lens makerâ€™s formula). They should be able to do this on their own, possibly with a little guidance.
Phase 2
Ask them to use a combination of ray tracing and mathematical manipulation to find out what happens when images are formed using a combination of two lenses.
Phase 3
Discuss how a convex lens can be used as a simple microscope. Take some time to discuss angular magnification, and comparing it with lateral magnification.
Discuss how a combination of two lenses can be used to construct a compound microscope and a telescope. Discuss both of these in detail, and in parallel, drawing out similarities and differences.
The Closure
Ask students to draw a table, listing the different kinds of images formed in different situations. This will help them review the chapter, organize their thoughts and will also act as self-assessment.
Activities
Give students as many kinds of lenses and mirrors, ask them to identify them as convex or concave, and find their approximate focal lengths.
Ask students to find out what kind of images are formed by different kinds of lenses and mirrors in different situations by practically forming images on screens.
The students can make a small microscope and telescope, using appropriate lenses. This will help them in understanding the difference between the two.
Assessment
Group discussions, along with individual numerical problem solving can be used to assess the studentsâ€™ learning experience. Lively discussions on conceptual problems should be encouraged.