A convex lens and a concave lens, each having same focal length of 25 cm, are put in contact to form a combination of lenses. The power in diopters of the combination is :
- 25
- 50
- infinite
- zero
The Correct Option is D
Solution and Explanation
PC = P1+P2P1 \(\rightarrow\) Power of convex Lens P2 =100f (in cm) =10025
=+4DP2 \(\rightarrow\) Power of concave lens P2=100−25 =−4DPC = P1+P2 = 4−4 = 0
Therefore, the correct option is (D): zero
Top Questions on Ray optics and optical instruments
Inductance of a coil with \(10^4\) turns is \(10\,\text{mH}\) and it is connected to a DC source of \(10\,\text{V}\) with internal resistance \(10\,\Omega\). The energy density in the inductor when the current reaches \( \left(\frac{1}{e}\right) \) of its maximum value is \[ \alpha \pi \times \frac{1}{e^2}\ \text{J m}^{-3}. \] The value of \( \alpha \) is _________.
\[ (\mu_0 = 4\pi \times 10^{-7}\ \text{TmA}^{-1}) \]- JEE Main - 2026
- Physics
- Ray optics and optical instruments
- A collimated beam of light of diameter 2 mm is propagating along the x-axis. The beam is required to be expanded into a collimated beam of diameter 14 mm using a system of two convex lenses. If the first lens has focal length 40 mm, then the focal length of the second lens is_____ mm.
- JEE Main - 2026
- Physics
- Ray optics and optical instruments
- In a microscope, the objective has a focal length \(f_o=2\) cm and the eye-piece has a focal length \(f_e=4\) cm. The tube length is 32 cm. The magnification produced by this microscope for normal adjustment is_____.
- JEE Main - 2026
- Physics
- Ray optics and optical instruments
- In parallax method for the determination of focal length of a concave mirror, the object should always be placed:
- JEE Main - 2026
- Physics
- Ray optics and optical instruments
\(XPQY\) is a vertical smooth long loop having a total resistance \(R\), where \(PX\) is parallel to \(QY\) and the separation between them is \(l\). A constant magnetic field \(B\) perpendicular to the plane of the loop exists in the entire space. A rod \(CD\) of length \(L\,(L>l)\) and mass \(m\) is made to slide down from rest under gravity as shown. The terminal speed acquired by the rod is _______ m/s.
- JEE Main - 2026
- Physics
- Ray optics and optical instruments
Questions Asked in NEET exam
- Given below are two statements:
Statement I: The DNA fragments extracted from gel electrophoresis can be used in the construction of recombinant DNA.
Statement II: Smaller size DNA fragments are observed near the anode while larger fragments are found near the wells in an agarose gel.- NEET (UG) - 2025
- Molecular Biology
Given below are two statements:
Statement I: Transfer RNAs and ribosomal RNA do not interact with mRNA.
Statement II: RNA interference (RNAi) takes place in all eukaryotic organisms as a method of cellular defence.
In the light of the above statements, choose the most appropriate answer from the options given below:- NEET (UG) - 2025
- RNA interference
- Given below are two statements: Statement I: In the RNA world, RNA is considered the first genetic material evolved to carry out essential life processes. RNA acts as a genetic material and also as a catalyst for some important biochemical reactions in living systems. Being reactive, RNA is unstable.
Statement II: DNA evolved from RNA and is a more stable genetic material. Its double helical strands being complementary, resist changes by evolving repair mechanisms. In the light of the above statements, choose the most appropriate answer from the options given below:- NEET (UG) - 2025
- Structure of DNA and RNA
- A microscope has an objective of focal length \(f_o = 2\) cm and an eyepiece of focal length \(f_e = 4\) cm. The tube length of the microscope is \(L = 40\) cm. If the distance of distinct vision of eye is \(D = 25\) cm, the magnification in the microscope is:
- NEET (UG) - 2025
- Optical Instruments
- Two gases A and B are filled at the same pressure in separate cylinders with movable pistons of radii \(r_A\) and \(r_B\) respectively. On supplying an equal amount of heat to both the cylinders, their pressures remain constant and their pistons are displaced by 16 cm and 9 cm respectively. If the change in their internal energies is the same, then the ratio \(r_A / r_B\) is:
- NEET (UG) - 2025
- Thermodynamics
Concepts Used:
Spherical Lenses
Lenses that are made by combining two spherical transparent surfaces are called spherical lenses. In general, there are two kinds of spherical lenses. Lenses that are made by joining two spherical surfaces that bulge outward are convex lenses, whereas lenses that are made by joining two spherical surfaces that curve inward are concave lenses.
Properties of Convex lens:
- In this, the lenses are thicker in the middle and thinner at the edges.
- They have a positive focal length.
- It intersects the incident rays towards the principal axis
- These lenses are used in the camera, focus sunlight, projector microscope, simple telescope, overhead projector, magnifying glasses, etc.
Properties of Concave lens:
- These lenses are thinner in the middle and thicker at the edges.
- They have a negative focal length.
- It parts the incident rays away from the principal axis.
- These are used in the glasses, spy holes, some telescopes in the doors, etc.