conditionally hiding & unhiding image folders in android

The following is my requirement: I have a checkbox in my activity. If the checkbox is checked, I will add a '.nomedia' file to hide all the photos in my folder. If the checkbox is unchecked, I will delete the '.nomedia' file, to display all the photos in the folder. My problem is, though the creation/deletion of '.nomedia' file is successful, the hiding/unhiding of the images are not happening in the android gallery app. How can I force the gallery app to show/hide the folder contents according to my checkbox state?

The following is my code:

CheckBox checkBox = (CheckBox) findViewById(R.id.checkBox1);
 if (checkBox.isChecked()) {
                                hideFolder();                                    
                            } else {
                                unHideFolder();                                    
                            }

    private void hideFolder() {
        File targetDir = new File(Environment.getExternalStorageDirectory().getAbsolutePath() + CONSTANTS.MYFOLDERPATH);
        File noMediaFile = new File(targetDir, ".nomedia");
        try {
            if (!noMediaFile.exists()) {
                noMediaFile.createNewFile();
            }
        } catch (IOException ex) {
            ex.printStackTrace();
        }
    }

    private void unHideFolder() {
        File targetDir = new File(Environment.getExternalStorageDirectory().getAbsolutePath() + CONSTANTS.MYFOLDERPATH);
        File noMediaFile = new File(targetDir, ".nomedia");
        if (noMediaFile.exists()) {
            noMediaFile.delete();
        }
    }